The Reagan administration holds that a communist threat from Cuba, the Soviet Union and Nicaragua is spreading among Central America's 23 million people. It wants Congress to increase aid and arms for U.S. friends in the mountainous, tropical region. Here is a primer on the six republics involved:

* Costa Rica: President Luis Alberto Monge has a solid majority in this strongly democratic country the size of West Virginia. He wants a multinational force to guarantee his border with Nicaragua, even though Costa Ricans supported the Sandinista overthrow of dictator Anastasio Somoza in 1979.

Now they are unhappy with Nicaragua's leftward swing. But Monge doesn't want to be a U.S. cheerleader and twice has asked Nicaraguan anti-guerrilla leader Eden Pastora to leave the country. Monge also wants more arms for his 7,000 Civil and Rural Guards and financial help for his ailing economy.

About 15,000 Americans live in Costa Rica, where the average per capita income is $2,300 a year.

* El Salvador: Interim President Alvaro Magana, handpicked by the army with U.S. help last year, shares power with right-of-center moderates and with extreme right-wing elements that seek a free hand against persistent leftist guerrillas. The 6,000 rebels are split into several factions in mountain strongholds around this Massachusetts-sized country, and their effort is backed by Cuba and Nicaragua.

Well-trained guerrillas have scored numerous military successes in recent weeks and now control or actively contest much of the country. President Reagan seeks more U.S. arms and expanded U.S. training in guerrilla techniques for Salvadoran Army troops. Reagan has proposed the training take place in Honduras, but Congress fears that U.S. involvement in the conflict will escalate.

Magana plans elections in December but leftists want negotiations instead, saying elections will be fraudulent. Meanwhile, tourist resorts sit empty in a wrecked economy, where the per capita income is less than $650 a year.

* Guatemala: President Efrain Rios Montt, a colorful general, took power in a coup last year, promising to end guerrilla violence that has plagued this Tennessee-sized nation since 1962. Using severe repression, he has pacified rural areas. He lifted the state of siege in March and plans assembly elections by the end of the year.

President Reagan, who says Rios Montt got "a bum rap" on human rights issues, has proposed a $6 million military spare parts sale, which would be the first U.S. arms sale to Guatemala since 1977.

Colorful Guatemalan cottons and spectacular Mayan ruins used to bring lots of tourists to the country, which has had Central America's strongest economy and is the only one with its own oil deposits, but now the per capita income is down to $1,100 a year.

* Honduras: President Roberto Suazo Cordova was elected president of this Ohio-sized country in 1980 following 10 years of military rule, but continued absolute control of the army makes Defense Minister Gen. Alvarez Martinez an independent force.

With the region's strongest air force, Honduras is planning to take advantage of a U.S. offer to build four to six new airstrips to help control Salvadoran guerrillas who Honduras and the United States say are infiltrating men and arms through Honduran territory.

U.S. funds targeted to help El Salvador also will build an "austere" regional training base in Honduras where 2,400 Salvadoran soldiers will get U.S. training by December.

U.S. covert funds are backing anti-communist guerrillas who receive supplies via Honduras to attack Nicaragua. Nicaraguans occasionally chase these guerrillas back over the border into Honduras, where they are headquartered, and Alvarez has asked for a U.S. promise to send troops if Nicaragua should invade. No such promise is reported to have been made.

Per capita income in Honduras averages $823 a year.

* Nicaragua: The Sandinista revolution that ousted dictator Anastasio Somoza on July 19, 1979, has become increasingly reliant on non-U.S. sources of assistance, including the Soviet Union and Cuba, as American economic and diplomatic doors slammed shut.

Daniel Ortega Saavedra, the only Sandinista commander on both the three-member ruling junta and the nine-member Sandinista National Liberation Front directorate, says the Marxist-oriented government will hold elections in 1985.

In early 1981, the U.S. administration decided to provide covert aid to a clandestine anti-Sandinista commando unit, officially to interdict arms supplies from Nicaragua to the Salvadoran guerrillas.

That force of "contras" is now at 7,000 men, according to CIA briefings to Congress, which passed the Boland amendment in 1982 outlawing covert funds "for the purpose of overthrowing the government of Nicaragua." The administration justified continuation of aid on grounds that the funds were being used only to sever Salvadoran supply lines.

Now two House committees have approved further amendments that would ban all covert aid to any group attacking inside Nicaragua,replacing it with open aid to friendly governments in the region for the purpose of cutting the flow of arms.

The size of Iowa, Nicaragua has a restive minority of Miskito Indians on its lowland Atlantic coast and an annual per capita income of $800.

* Panama: The death of former National Guard strongman Gen. Omar Torrijos in a 1981 plane crash left a leadership vacuum in this tropical nation the size of South Carolina. Gen. Ruben Dario Paredes, head of the guard, recently announced he would abandon that power base in August to run for president next year in a return to full civilian government.

The Panama Canal has set new records in tonnage, ship transits and revenues since the Panamanians took over the Canal Zone from the United States under the treaties of September, 1977. The switch has alleviated strong anti-American feeling among the citizens, many of whom continue to press for removal of the U.S. military School of the Americas and U.S. Southern Command headquarters here.

Panama is a strong international banking center. Its per capita income is $1,850 a year.