Action-to-action is the combination of shots from multiple cameras that is combined in editing to make a scene seem to take place in one place and time. It's achieved through cutting these scenes together so that the division between them is almost nonexistent to the audience. It can also be used to increase tension.
This clip from Jean Cocteau's La Belle e Bete uses action-to-action to simulate that the arrow targeting is coming from the same person that shot it. It compresses time, and allows the audience to assume that these separate shots happened at once instead of over the span of a multiweek shoot.
This clip from the Ballad of Narayama uses two action-to-action shots. The first is the Hunter shooting after the rabbit. The second is the hawk taking the rabbit and flying away.
This clip from Werner Herzog's Fitzcarraldo uses action-to-action both in the compressing sense (in the way that it shows something that was shot at different times composed into shot), but it also uses elliptical editing (as it shows a passage of time).