Computation is what you do when you calculate something, like a simple math problem or something involving quantum mechanics or — worse — even your taxes.
People have invented all kinds of machines to help with computation, from the abacus to the computer, and, in fact, when computers were first invented (and when one computer could easily fill a large room), using a computer was called computation. Much of what made computers so revolutionary was that in a relatively short time they could do a complex mathematical computation that would take a human many lifetimes to do.