trim
To fine-tune the edit point between each pair of clips after you’ve assembled your clips in the timeline. Any time you make a clip in a project longer or shorter, you’re trimming that clip. However, trimming generally refers to precision adjustments of anywhere from one frame to several seconds.
Note: No matter how you trim or make other edits in iMovie, the underlying media is never touched. The trimmed or deleted pieces of clips are removed from your project, but the clips in the library and the corresponding media files on your Mac or storage device remain unchanged.