Quite simply, the U and L on your aligner sets stand for “Upper” and “Lower”, so you know which tray is designed to fit where in your mouth. It’s important you put them in the correct place otherwise you can push your teeth in the wrong direction and set your treatment back.
If there’s anything else with your aligners that you aren’t clear on once they’ve been shipped to you, you can get in touch with the Customer Support Representatives who will be happy to help you make sure you’re using your aligners correctly.