Apple Final Cut Pro 5 - Manual (Page 1299)

512 Part III Output
About Safe Real-Time Playback
The Safe RT mode guarantees that effects are played back at the quality and frame rate
you specify, and that no frames are dropped during playback. If Final Cut Pro
anticipates that your computer won’t be able to perform all the effects calculations in
real time, the render status of that portion of the Timeline becomes red, which means
you have to render before you can play back.
Safe RT is useful when you are outputting your final movie to tape, because it ensures
maximum quality. In many cases, you may have to render before output to tape. By
default, the Edit to Tape and Print to Video commands use full quality.
Red video render bars indicate
rendering is required.