"Bulky five" is a term from the game of Go
). It refers to this basic shape, essentially a square with an extra space sticking out:
The reason I don't draw the surrounding board is that it's rare to see this shape formed by a group of stones. Rather, it usually occurs as a negative space, that is, a hollow big eye in the middle of a group of stones, as shown here (edge of the board marked by ###):
(No stone at a, but see below.)
Here, Black's (x) group has a single big eye, in the shape of a bulky five.
The reason it's important to know about the bulky five is that it's the only 5-point big eye which is not unconditionally alive. Rather, it is what is known as unsettled. That is, if Black has the initiative, he can play at the vital point a, and divide his big eye into two eyes and live (see my writeup on the eye node for an explanation of why two eyes are needed to live). If it is White's turn to play, she can play there herself, thereby preventing Black from ever making two eyes. In this case, Black will be unconditionally dead.
One way of killing a group is to try to make its big eye almost filled with a group of your own stones in the shape of a bulky five, as shown in this diagram:
White has formed a bulky five of her own stones in Black's big eye. As soon as the outside liberties (marked by commas) are cut off (which White can do at her leisure), Black will be forced to capture at the point marked with an asterisk. Black will be left with a bulky five eye, and since it's White's turn, she will play on the vital point and drive the final nail into Black's coffin.