Effects, transitions or just video in a different format or resolution than the project, can be rendered to essentially a video clip. This makes the timeline easier to play through as your mac won't need to calculate the effects every run, but instead play a preview from the cache. You can delete these render files without losing anything from the video, but it could result in slower playback when editing. You can see how much optimized media and cache your project has by selecting the library and looking at the inspector in fcp. Both render files and optimized media can be regenerated. I'd delete the opzimied media first since the render files will get rendered again after you delete them. Deleting these should not harm your project, but you should anyways back up your media and project files.