How to play Go: The rule of seki
Consider the situation below
This situation is called a seki. Neither player wants to play another move in this area. Let's see why.
Firstly, it is clear that Black does not want to play
Adding another stone costs Black a liberty and White captures all the Black stones immediately.
However, what if white plays first?
Black captures the White stones and now wherever White plays Black can make a living shape
Neither player wants to play again so this set of stones will remain until the end of the game (presuming the white stones on the outside live).
When counting territory, this seki situation scores no points for either side. No stones are taken off and the surrounded intersections are ignored.
Sekis occur infrequently but playing inside what your opponent thought was his territory and turning it into a seki can be a game winning move.