Screenshot - what is this on my layer?
That means there's extra media after the end of the clip.  If you hover the mouse over the intersection of the semi-transparent part and the opaque part, it will turn into a double arrow &lt;-->.  If you then click and drag to the right and play back the file, you'll notice extra media appear as part of your composition.  I.e. something more will happen in the video.  Drag it to the left, and the clip will disappear when the playhead hits that part of the timeline.  The transparency just lets you know that whatever recorded the media file (a camera, for example) recorded something more than what you're seeing play back in the viewer.  In other words, you could potentially extend the edit.