Screen is super-useful because it allows you to run terminal commands without needing to have your terminal window open.
Think of a screen as a session: you can open a screen, start a script, and then detach from it. The script will keep running in the background, but you can keep doing whatever you were doing without affecting the script. More significantly, if you want to be able to step away from your terminal, or close your laptop, or let a task run overnight without needing you to be connected, use a screen.
This quick tutorial will walk you through the basic commands necessary to get a screen working without worrying about what's going on behind the scenes.