Alternating views of one action with views of another. It is a rather loose term, used to describe two different situations.
Within a scene, the filmmakers may cut from one part of the action to another. To present an interrogation, for example, one might cut frequently between views of the questioner and those of the prisoner.
Or one might cross-cut between actions taking place in two different locations. The textbook example is the climax of Birth of a Nation: views of the heroine in peril alternating with views of the Ku Klux Klan riding to the rescue. In this example, the two lines of action are going to meet, but one can cross-cut between actions that do not.