Two Eyes

Groups of stones can be captured once they are completely surrounded. The black stones below are in such a predicament - nowhere to escape and awaiting capture by white.

static . . . . . . . . . . . . . . . . . . . . + O O O + . . . . O X X X O . . . . O X T X O . . . . . O X X O . . . . + . O O + . . . . . . . . . . . . . . . . . . . . --- triangle: T

Black can play freely once again. White will not answer, but be on the lookout for illegal moves.

freeplay . O . . . . . . . . . . . O O . . . . . . . . . . . . . . . . . . . . . . . . . . . + . . . . . + O O O . . . . . . . . . . O . . . . . . . . . . . . O . . . . . O O O + . . . O O O . . . O . O . . . . . . . . . . O . O . . . . . . . . . . + O O . . . + . . . . . . . . . . . . O O O O . . . . . . . . . O . . . . . . . . . . . . O . . . --- color: black

Two eyes

Consider the case below.

static . . . . . . . . . . . . . . . . . . . . + O O O O . . . . O X X X X O . . . O X T X T X O . . . O X X X X O . . + . O O O O . . . . . . . . . . . . . . . . . . . --- triangle: T

While the black stones are surrounded, white cannot capture them. There are still two spaces required, both of which are illegal moves.

We call separate internal spaces eyes. Once a group makes 2 eyes, it cannot be captured. We can call the group unconditionally alive.

Try to capture the white stones on the board once again.

freeplay . O O . . . . . . . . . . O . O . . . . . . . . . . O O . . . . . . . . . . . . . . + . . . O O O . . . . . . . . . . O . O . . . . . . . . . . O . O . . . . . . . . . + O O . O . . . . . . . . . . O O O . . . . . . . . . . . . . . . . . . + . . . . . + . . . . . . . . . . . . . . . . . . . O O O O O . . . . . . . . O . O . O . . . . . --- color: black

Could you capture them?

I bet you could not! They are all unconditionally alive because they have 2 eyes.