Your UID is a long string of numbers that uniquely identifies your Discord account.
Staff need it to look up your ban. Follow the steps below, or watch the video.
Or follow these steps:
Open Discord Settings.
Click the gear icon ⚙️ near your username, bottom-left of the app.
Go to Advanced.
In the left sidebar, scroll to Advanced under App Settings.
Turn on Developer Mode.
Toggle Developer Mode on.
Copy your User ID.
Close Settings, right-click (or tap-and-hold on mobile) your own username/avatar anywhere in Discord, and click Copy User ID.
Paste it into the appeal form.
Head back to the appeal form and paste it into the UID field.