It can occur that you can capture a stone, while your opponent can immediately recapture your capturing stone. Here you see an example. White captures with 1 a black stone, but black can immediately recapture stone 1 with 2.
In this sequence there is repetition: the same situation is coming back every time. This situation is called ko.
To prevent this from going on forever the ko-rule has been introduced: if your opponent captures one stone and you can recapture the stone that was played immediately after that (bringing the same situation back on the board), then you are not allowed to recapture immediately. You have to wait at least one turn before you are allowed to recapture your opponent's stone.