Ukemi is the art of falling. In aikido, this involves learning to take yourself from a state of balance through a state of unbalance right back to a state of balance. This usually involves rolls, but there is also a more dramatic form of ukemi called a breakfall which is used in drastic cases (a very powerful throw, or the chance of breaking oneself into tiny pieces). In aikido, ukemi generally flows very nicely and is also useful in daily life at times. (hey, it's saved myself from getting hurt a time or two...)