What is the line on the bottom of the screen?

You don't. That is the reimagined home button, which you slide up to either close an app or go back to your home screen. It is built into iPadOS and is not removable.

The black line to which you refer, seen only on models of iPad that don’t have a physical Home Button, is an element of the User Interface. It cannot be suppressed.

Think of the line as a “handle”; Touch the handle and swipe to the right (or left) to switch between recently used Apps. This is an alternative to the four-finger swipe gesture, also used for fast switching between recently used Apps.