The article grants Madrid the right to “give orders” to “all authorities” of a regional government if said government “doesn't comply with the obligations of the Constitution or other laws it imposes, or acts in a way that seriously undermines the interests of Spain.” The article can only be invoked if an absolute majority of the country's Senate approves its use. At that point, Madrid can adopt all “necessary methods” to force a regional government to comply to protect “said interests.”
(If you’re curious, here’s the full text, in English: “If a self-governing community does not fulfill the obligations imposed upon it by the Constitution or other laws, or acts in a way that is seriously prejudicial to the general interest of Spain, the government, after having lodged a complaint with the president of the self-governing community and failed to receive satisfaction therefore, may, following approval granted by the overall majority of the senate, take all measures necessary to compel the community to meet said obligations, or to protect the above mentioned general interest.")
It’s hard to know what that will look like in practice, since Article 155 has never been invoked before. As my colleagues explained:
After the Senate invoked the never-before-used Article 155 of Spain’s 1978 Constitution, the central government could move swiftly to remove the Catalan regional president, suspend his ministers and assume authority over the region’s public media, police and finances.
How did Article 155 come to exist in the first place?
To understand that, it helps to have some history. Spain began as several autonomous regions, and a large part of the country's history is really the story of bringing disparate societies together. In the 20th century, the drive toward regional autonomy triggered the 1931 civil war. And when Francisco Franco seized power in 1939, he responded to the push for regionalism with a policy of limpieza, or cleansing. During his reign, Franco ruthlessly pursued a unified Spain, canceling autonomy agreements such as the one with Catalonia, jailing supporters of secession and making it illegal to speak Catalan or wave the Catalan flag. Franco even forbid Catalans from celebrating Diada, the region's holiday.
When Franco died and Spain began its transition to democracy, one of the first things that happened was the restoration of some degree of autonomy to regions like Catalonia. This didn’t sit well with Spain’s right wing, which responded “swiftly and violently to the reintroduction of regional autonomy, which was perceived to be a threat to national unity,” as Foreign Policy explained.
The compromise, hammered out in the country's 1978 constitution, was a system of 17 self-governing regions. Each region set its own terms with the federal government. And Article 155 was seen as a check to that power, a way for Madrid to protect its interests. It was a check that was never actually used.
Never, until now.