Linux (and Unix in general) restricts access to the system through login/logout. Login is a process for a user to authenticate himself/herself to the system. Users in the system is represented by user accounts and protected by user password. To access the system, identify yourself with the username and give your password to the system. Upon successful login, you'll be given a shell (at which you can issue commands, such as ls, rm,...).
Remember to logout when you finished your session, otherwise bad people may use your session to access the system (at least your own files/data are under risk). This is particularly true if you login as root!!