pushes White along, taking territory and also reducing the liberties on the white stone. This is a typical pushing play, of the kind that can lead to a pushing battle. In Japanese it would be oshi.
Pushing from behind is considered to be disadvantageous.
(Sebastian:) Is my impression correct that we can't call it a push until we see what White's answer is? Would it not be a push if e.g. at a?
Push in, push out, push through relate to a complex of different basic patterns, the Japanese de and degiri (q.v.).