what is masa?

Sam's answer is right on. When corn made it's way to Europe from the post-columbian New World it took them decades to get the lime part right and so the nutrients went missing.

It's what is mixed with water to make corn tortillas, Pastries, and the dough filling for tamales. Masa Harina as mentioned. Unlike cornmeal, it's been soaked in a lime water solution and dried---this ancient process release the nutrient niacin allowing it to be absorbed by humans.
Masa is also blanket term in Spanish for "dough", but when you see it used here, most of the time it means Masa Harina. (corn flour).

It's corn flour or the dough from corn flour. Sometimes you see the flour called Masa Harina.