stretch to gain a liberty
Alex: When there are no adjacent stones and it does not form an empty triangle, a simple stretch (nobi) tends to gain (at least) one liberty in sente (in a capturing race) or two in gote. This is because it creates three new liberties for the group (the circle marked points shown here), while using up one itself. The opponent will only be able to remove one on his next move (assuming removing a liberty is the correct thing to do), for a net gain of one.
This is important for tactical fights, which gives us principles like "extend from a crosscut". It is also quite often crucial in capturing races (semeai), however, and something that, although obvious when pointed out, is easy for beginners to overlook.
Since the opponent may be able to play at the point himself and thus remove a liberty while preventing you from gaining one, it is usually important to play these stretches before rushing to remove the opponent's liberties. Also, it is important to look for and account for such possible stretches ahead of time when reading out a capturing race and deciding if it is winnable.