All visual objects (images, videos and clips) that have been dropped onto the Stage, will also appear in the Timeline. Every object represents a 'layer'. When working with multiple layers, it can become unclear to work in a very detailed way on the Stage.
Therefore you can simply hide and unhide parts of your project when working in the Editor. In the Timeline, just click the 'Eye thumbnail' next to the object or clip that you would like to (un)hide:
You will see the object or clip will disappear from the Stage, but will remain visible in the Timeline.
Make sure before rendering your video, to unhide all objects, as they will be rendered along with all other objects and will be visible in your final downloaded or published video.