The Trim command in the QuickTime app allows you to remove unwanted parts of a video or audio file. Here's how it works:

1. Open the video or audio file in QuickTime.
2. Use the playback controls to navigate to the point where you want to start trimming.
3. Click and drag the yellow playhead to the start point of the section you want to keep.
4. Choose Edit > Trim (or use the keyboard shortcut Command + T).
5. QuickTime will create a new video or audio file with only the selected section.
6. Save the trimmed file with a new name or overwrite the original file.

Note that the Trim command is non-destructive, meaning that the original file is not modified in any way. QuickTime simply creates a new file with the trimmed section. This allows you to experiment with different trims without affecting the original file.