Match on Action

Match on action basically means that an action that continues through different shots has to go on at the point in a new shot, where it was cut in the shot before.
For example: If you show a man opening a door and in the first shot, filmed from outside the room, he opens the door half, and you change the camera perpective to inside the room, the door must also be half opened in the new shot.
Match on action is important for the audience to understand the action happening.

You can see an example in the video above at 0:18, when the shot changes from Close-Up to Mid Shot as the girl flips a page.