After you’ve roughly assembled your clips in chronological order in the Timeline, you begin to fine-tune the cut point (or edit point) between each pair of contiguous clips. 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. If you’re adjusting clip durations by much larger amounts, you’re still trimming, but you may not be in the fine-tuning phase of editing yet.
No matter how you trim or make other edits in Final Cut Pro, the underlying media is never touched. Trimmed or deleted pieces of clips are removed from your project only, not from the source clips in your library or from the source media files on your hard disk.
Tip: You can see a “two-up” display in the Viewer as you trim edit points in the Timeline. For more information, see Show detailed trimming feedback.