That tends to happen if you've lost your media but have already started editing. Those "items" are probably referring to clips that were made from the master clips, the ones you're trying to recapture. The message is trying to tell you that those clips are referencing the one piece you're trying to recapture. Most of the time, you can simply click "Relink" and everything should go fine. If you're not recapturing, though, then we'll need more information before we can tell you what might be going on.