A VRChat account is an official account, most likely created from the official VRChat web page.
An official VRChat account is required for you to create avatars and worlds for the platform. Adding on to this information, you need to use your VRChat account in order to be eligible for trust. Otherwise, your rank will never change.
(If you have random letter and/or number spam after your username, it's because you're not using a VRChat account. Try creating one and linking your existing account to it.)