Only what belongs to this account: username, email, password, and the devices that are signed in.
These three belong to your account in the database. My List and your watching progress are stored under the same account, so signing in somewhere else lets you pick up where you left off.
Changing the password signs out every other device on this account.
Every device holding a live session for this account. Eject anything you do not recognise — that device has to sign in again.