There is always at least one user in the system - Administrator. If necessary, you can add more users. This lets you see who created a record, protect your own records from changes and hide from other users (see access restrictions). It makes sense to set a password for each user including Administrator.
The administrator account has a special status. Administrator can see all records (including hidden ones) and edit records protected from changes. The administrator account cannot be deleted.
You can drag one user into another to create a group. Access restrictions of a group apply to all users in that group, but certain user can have different restrictions - this settings have higher priority.