When you experience regression, you "go back" in some way. If you've been trying to break your sugar habit but one day eat several pieces of cake, that's regression.
Remember that the suffix -ion is added to a verb to make it into a noun, making regression the noun form of the verb "to regress." We can find regression, meaning "the act of going back," as far back as the late 1300s, from the Latin regressus, which means "a return." When you see all your old friends (and enemies) at a school reunion, you might experience regression and start acting like your younger self.