White bumps into the Black stone. A natural local response is usually to stretch to a or b. A common term for this move is butting play.
The Japanese term is tsukiatari.
The bump is not a very usual move, but it has its uses.
Example 1: Kitani joseki
The bump is occasionally used as a situational move? for preventing White from connecting underneath at a.
(unkx80: A better example is requested.)