Adding a timecode overlay
Timecode has been around for decades and is a tool that easily allows an editor to identify each and every frame within a clip. Timecode essentially gives every frame a name. They're often overlayed in rushes and rough cuts so a director or producer can say, "Hey, can you change the transition at 00:34:52:17?" rather than "Hey, can you change the transition that's about a half an hour in, between the shot of the monkey shooting lasers out of his ears and the shot of the couch falling from the sky and landing on the sad clown?"
FCPX can easily apply a timecode reader to appear over your video so that it can be viewed right in FCPX's Viewer (if your director/producer is right there with you) or in any exported version of the video.
Getting ready
You can add timecode at almost any point in the editing process. Usually, however, it's easiest to do it towards the end, right before you are either going to export a version of your video or show it to someone right there in...