Have you ever watched a movie or a show and noticed that the camera sometimes cuts from one shot to another while someone is moving or doing something? That's called cutting on action!
Basically, cutting on action means that the video editor picks a specific moment in a scene where there is some movement or action happening and cuts from one shot to another at that exact moment.
This makes the scene feel smoother and more seamless. It also makes it easier for the viewer to follow what's happening in the scene because the action is continuous even though the shots may be different.
Think of it like jumping from one stepping stone to another - you don't want to jump in the middle of the water or you might fall! Cutting on action helps the editor make a smooth transition from one shot to the next, just like jumping from one stepping stone to the next helps you cross a river.