Final Cut Pro X: About background rendering
Rendering is the process of creating temporary video and audio render files for segments of your project that Final Cut Pro can’t play in real time. When you add effects, transitions, generators, titles, and other items that require rendering for playback at high quality, the render indicator (a light gray dotted line) appears below the ruler at the top of the timeline.
By default, rendering begins in the background 5 seconds after you stop working and moving the pointer in Final Cut Pro. You can turn off background rendering or adjust this setting in Final Cut Pro preferences (see Playback preferences).
You can also manually control rendering for a project or a portion of a project, and you can delete render files for one or more projects, events, or libraries to free up storage space. For more information, see Manage render files.