(a) Trimming a part of a movie means to hide a part of a video clip. When we trim a clip, the trimmed part of the clip is not actually removed from the source file; it's just hidden so the trimmed part doesn't appear in our project or published movie.
(b) The preview monitor enables us to view individual clips or an entire project. By using the preview monitor, we can preview our project before publishing it as a movie. We can use the buttons underneath the preview monitor to play or pause a clip, or to advance or rewind a clip frame-by-frame.