There are several ways to login as a user or change to another user.
The simplest way to login is to actually login. Just use the monitor and keyboard, or connect with a terminal (ssh) over the network.
Most installations have a root user for maintenance, and at least one normal user for daily use.
Now if you are logged in as a normal user and decide to do some maintenance, you can logout with <ctrl>d and then login as root. Another option is to stay loggedin as the normal user and login on another session as root. On hardware another session is created with <alt>Fn where n is function key 1-12. With a terminal application a second session has to be started.
The sudo command is for executing one command as root. From within the session of the normal user execute:
It will ask for the password of the currently loggedin normal user.
Another option is to become root for a while, while still being loggedin as a normal user
The password of root will asked, and after that you are in a session of user root. To leave the root session type <ctrl>d, and you are back in the normal user's session.
The other way around, if loggedin as root, su can be used to start a normal user's session. This can be useful for testing if that user is allowed to do the things you expect.
No password is asked, and to leave the user's session, use <ctrl>d.