The su command allows the user to switch to root.

grumpy$ wall Hello, World!
wall: command not found
grumpy$ su
# wall Hello, World!
Broadcast from root@grumpy (danamals):
Hello, World!

If the user does not have root on the host:

grumpy$ su
su: incorrect password

No matter what password you enter, if you do not have root on the host, it will not allow you to become super user.