Every chess engine calculates a certain number of moves in advance. After all plausible move possibilities have been analysed, the program starts the calculation anew, but then calculates one half-move deeper. Every chess engine distinguishes during the search between sensible and senseless moves. The moves that the program considers sensible are analysed more deeply; this is also called selective search.
Examining the remaining moves is called full search. Almost all chess engines display both the selective and the full search in the analysis window. You should know what a "half-move" means in computer chess terminology: a half-move is one move by either White or Black. A move thus always consists of two half-moves.