So let's start with the basics.
A monarch is someone who is head of state, typically a king or queen.
Absolute is refferring to absolutism.
I could not explain what it is so I got the definition offline:
"The acceptance of or belief in absolute principles in political, philosophical, ethical, or theological matters."
The definition of an absolute monarch:
The definition of an absolute monarch is a king or head of state that rules in his own right.
Have a nice day!