Trimming is the process of adjusting where two clips touch. (The point where they touch is called the edit point.) To me, trimming makes the difference between a home movie and a professional film. Trimming is the process of finding that one spot where two clips transition so smoothly that the viewer barely notices that an edit has occurred.
That’s the good news. The bad news is that you can spend 30 minutes trimming to create the perfect edit; then, when you play it for your friends, it goes by so smoothly that they don’t even notice it.
But the audience notices, because a well-trimmed video captures their attention and holds it in the story, while bad trims keep jarring them back to reality.