I think it's the little plastic you put into a hole in the wall after you drill it. It's so the screw you place in the wall is securely set and won't move. Cool name!
Antonio López de Santa Anna was the president (eleven noncontiguous terms) of recently independent Mexico in the early 1800s and wanted to build a monument to the Revolution that started in 1811 and secured that independence. He had the government put up a plinth for that monument in the central plaza where the National Cathedral and Presidential Palace sat, but it was never finished. Santa Anna was too busy losing Texas to the slave power; the Anglo Texans rebelled because Mexico had abolished slavery and they wanted some.
Now Mexican towns almost always have a central square with the local church on one side and the ayuntamiento (city hall) on the other. It usually has elaborate band stands, gardens, benches, and flagstone walking paths.
The central plaza in the capital of the Mexican Republic, officially the Plaza of the Constitution, came to be called by metonymy the Zócalo. The plinth itself was taken down in the 1850s, but the name stuck. In fact, the name spread through the country and now all the central plazas in the republic are called zócalos.