Liberties

A liberty is an empty point next to a block of stones. For example, this white block has three liberties, at A, B, and C:

A block is captured when it has no liberties. A block with a single liberty is said to be in atari.

Having just one liberty more than the opponent can be decisive. For example, the two blocks below are locked in a race to capture: White can play A and B to capture the three black stones, while Black needs to play C, D, and E to capture the three white stones. White will win this fight.