In Kdenlive, a project is made of clips. These can be whole audio or video files, or segments of files you add to the project.
Adding clips to the project
If you drag a file onto the timeline that is not in the project (for instance, if you drag a DV file straight from Konqueror to Kdenlive), then the file will be automatically added to the project for you.
You can add clips to the Project List yourself. Either use the Project List context menu (right click on the window), or the Project menu on the menu bar. Select 'Add Clips' to bring up a file dialog in which you can select the clip you wish to add to this project. As long as the clips are valid, they will appear in the Project List.
Deleting clips via the Project List
You can delete clips from your project by selecting 'Delete Clip' via the clip context menu (right click) or from the Project menu. When you delete a clip from the Project List, this will also remove any clips on the timeline that are using it.
Clean Project - removing unused clips
Use Clean Project to remove any clips from your project that are currently unused. This is great if your Project List has become cluttered and you want to get a concise view of which clips exist. Cleaning a project will not alter your final edit in any way --- the only clips that are removed are those that are no longer in use anywhere.