Final Cut Pro X render files use a lot of hard drive space. To recover some of this space, it's important to delete these files from time to time (especially for projects you're not currently working on).
Deleting render files is harmless, Final Cut Pro recreates them automatically when you open a project again.
Process for Deleting Final Cut Pro X render files:
1. Select a Project/Event/Library in the Browser by simply clicking on it so it's highlighted.
2. In the menu; choose File > "Delete Generated Project Files..." (or "Delete Generated Event Files..." or "Delete Generated Library Files...")
3. Choose Delete All Render Files and Click OK.
And that's it, your hard drive(s) will thank you!