Dia 1: Black to move in this position...	Dia 2: Would he decide to capture one white stone...	Dia 3: It would lead to this position...
But now it's white's turn...	Dia 4: White could decide to capture one stone...	Dia 5: Leading to the position of dia 1...
Dia 6: Endless...	You get the picture: this process could repeat itself endlessly. To avoid this to happen the game has a special rule, the ko (Japanese) rule. The ko rule prohibits that the same position (i.e. the whole board!) repeats itself.

Note 1: So in the above diagram sequence white 2 is prohibited because of this ko rule.

Note 2: Its not allowed to repeat the "whole board" situation so once one (or more) stones have been played elsewhere you're again allowed to take the ko. When it's important to capture (and win) the ko these "moves elsewhere" are usually threatening something so that your opponent has to answer and thus has no time to connect the ko. Such moves are called "ko threats".