Focus is how foggy or how clear an object appears on screen. 

In this still from Sidney Lumet's Dog Day Afternoon, Al Pacino is the only thing in focus while the portraits behind him and the teller counter are out of focus. This highlights that he is the subject, and acts as a visual cue to the audience. This technique is known as "shallow focus" where only one or a couple of objects are in focus at a time. 

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In this still from Francois Truffaut's 400 Blows shows everything in focus including the subjects and the scenery. This technique is known as "deep focus" and as digital film-making has risen to prominence it has become easier to do.