Rearranging your clips
This basic part of the edit — putting things in the right order — is very important, yet deceptively simple. FCP makes it hard to get this wrong, because it's hard to break up a clip by accident, and the Magnetic Timeline means that you won't leave any gaps unless you add them on purpose.
If you're new to editing, this probably doesn't seem too strange. You're putting clips in order to tell a story, so why would you want to leave gaps in that story? It's probably a little jarring for new editors to realize that simply rearranging clips is actually not a simple operation in most other non-linear editing systems (NLEs). One of the key innovations of Final Cut Pro was to make it easier for you to experiment with your edit, letting you rearrange your clips without worrying about breaking anything. If you're an experienced editor used to juggling clips up and down to avoid clip collisions across multiple tracks...