Say "Absolutely" to express that you completely agree with what someone has said.
You can agree with someone's opinion this way.
A: I've always said that you shouldn't hire anyone who you don't feel 100% certain about.
B: Absolutely. I totally agree.
You can also agree to a request:
A: Hey, can I talk to you for a minute?
"Absolutely" sounds even more definite than "yes". You answer "Absolutely" when you're really sure that the answer is "yes".