Focus refers to the attention of the audience (and the actors, too) on certain people on the stage. If there are two people, then the attention of the audience is mostly focused on the one that is presently speaking or acting. In scenes with more than two people, the question of focus is more important. In this case, usually at least one of the three or more will be in the background for a while. That means, while one or two people are in the limelight, the others make some unassuming routine tasks and do not speak or talk very quietly. The focus can be taken or received from others. The focus can be strengthened by presence and radiation.
It is important, that as a rule a person who is currently on stage should have the focus. This can also be a passenger or a prop.
If the people on stage do not pay attention to the focus at any given time, the scene appears - and not only for the audience - more or less unclearly structured and chaotic. A clear focus is also missing, when several actors speak loudly at the same time. If someone recklessly and continuously seeks the limelight, he will have the focus but will be labelled as a stage hog.