Absent means not there. If you absent yourself from class, you're marked as absent. If you have an absent parent, it means they don't live with you or take part in your care.
Absent can also describe a state of mind, meaning "not all there" or "lost in thought." If you are absent from class on a test day, you will have to explain your absence, unless your teacher is so absent minded she forgets and just gives you an A.