The marked stones show the basic situation. White 1 is the eye-stealing tesuji, taking away the potential eye at : a simple but very useful move. A Black play at instead would make an eye there for black.
The eye-stealing tesuji typically involves the following shape:
Variants of the tesuji can occur in simpler situations:
With in place, even after Black plays both , any one of a, b, c makes the eye false. White can play whichever is convenient and Black will have trouble defending them all.
An eye can also be stolen from this hanging connection:
Sometimes White can play adjacent to , here or at a; in this case Black cannot even threaten to make an eye there. If possible, this may give White better shape, but the usual tesuji may be the only way to steal the eye.
This is not much of a threat in isolation, but if there are a few other white stones about or if it can be played as a ladder-breaker or a ko threat so White gets more than one move in the area, it might lead to an attack on the entire black group, perhaps splitting it into two weak groups.
If the cut is something to fear for black (white stones along the left edge) may well gain something.