In making a video, you may be working with several different components like audio, text, animated emoji's, video clips. These components will each have their 'row' in the Timeline, labelled as Track 1, Track 2, Track 3 and so on.
As you progress with editing, you can add more rows, depending on how many elements you want to put in your video. With that, it can be bothersome to see them all at once, especially if you only need to work on one part at the moment.
In such cases, hiding some rows which you're not using, can clear the clutter on your Timeline.
To do that, go to the left side of the Timeline and go to the Track you want to hide and click on the "eye" icon. It should hide the row, and you can work on the others.
This move is temporary though, and it will only be in your editor. If the project is exported, all the rows or Tracks will be visible when opened.