One liberty = atari
A group with exactly one liberty is in atari — one move from capture. The skill here is not giving atari but seeing it: after every move, both players should sweep the board for any group down to its last liberty, theirs or yours, because an unnoticed atari is the most common way beginners lose stones. This white stone has already been fenced on two sides; two liberties remain, right and below, and either one finishes the job. What this lesson trains is recognition speed, not calculation depth — the ataris you fail to see are the ones that cost you. Black to play: put the white stone in atari.