THE CANCUN conference on world poverty and wealth threatens to be another depressing case of politicians talking past each other. Most of the gathered presidents and prime ministers will speak in terms of their own respective national experiences, and will be much exasperated when half the audience misunderstands them. President Reagan illustrated the possibilities last week when he adjured the poor countries to put their trust in free markets.

After all, look at the American example. "Few countries are less developed than we were when the original settlers arrived here," he observed. "They faced a wilderness where poverty was their daily lot and danger and starvation their close companions."

But those settlers also faced the planet's greatest reserve of first-class agricultural land, with a mild climate and phenomenally reliable rainfall. Americans rarely reflect that North America is the only continent whose people have not been repeatedly subjected to famine, and whose cities have never known bread riots. Free markets had a lot to do with the increase in American prosperity. But it is at least equally true that the stable and rapid increase of prosperity had a lot to do with the development of American freedoms.

In the Third World, where many countries have only two or three significant exports, the free market means the roller coaster of international commodity prices. Life is good when tin, or copper, or coffee is high--and then the economy goes into paralysis when they fall. That's why Third World governments keep bringing up ideas for worldwide price stabilization schemes. The industrial countries reply that those schemes can't possibly work. That's correct, unfortunately, but it also leaves people in the poor countries to the south with an impression that the principles of economic freedom serve them less well than their richer neighbors to the north.

Mr. Reagan doesn't hold out much hope for the increases in economic aid that the poor countries will claim at Cancun. But he offers one important promise--to keep markets open to the products of the developing countries. That, as he said last week, is "absolutely essential," but it won't be an easy promise to keep. As those countries industrialize, their products increasingly compete with industries here. Their steel, automobile parts, electrical goods and textiles all touch sensitivities in the United States.

Perhaps the best that can come out of Cancun is a tacit agreement to tune down the ideological rhetoric on all sides, and begin a conversation about the modest but practical steps that hard-pressed governments, both in the rich North and the poor South, think that they can actually achieve. Exhortation on economic theory will not be useful. Trade expansion may not be very exciting, but it has the merit of being possible. In a year when they don't really expect to do much more, governments-- beginning with Mr. Reagan's--have a special obligation to do that much.