Capital Buffer

Banks are required to maintain a certain amount of capital. But then, on top of that (just for paaddding!), they're required to keep some extra capital on the side as buffer.

It's kind of like when your mommy used to make you wear kneepads, a helmet and ten layers of clothes before she helped you mount your bike (no wheelies!)? All that extra padding (hopefully) helped and served as buffer against a fall. Capital buffer's that extra padding in the middle.