What is this and how can I get rid of it?

Are you referring to the work area in your timeline? What particular problem are you experiencing with it? You can change its length, drag it to another position in the timeline, right-click on it to set the beginning or end of it to the current position of the playhead and more. You should be able to remove it by clicking on the Timeline menu and unchecking "Enable Work Area." The work area is used to allow you to concentrate on specific aspects of your scene. Here's some information about it here: Weird Starting JUMP! | ElectroPuppet