Your Account ID (or Player ID) is your unique code that helps us identify you and your account. Cosmetics, XP, and data is stored in this ID, so it's important you don't share it publicly.
In order to help resolve your issue, we will likely ask for this number. Here's how to find it:
On the main menu, click the "Gear" icon.
Hit the "Data" tab.
Click the button at the button a few times to automatically copy and paste your ID.