"Having a ball" is an idiom for having a lot of fun, enjoying something a lot... almost euphoria. It's probably related to "ball" meaning "a social dance," and often refers to a something social. You might say "The grandkids were over last weekend and we really had a ball." You _probably_ would not say "I really had a ball reading Bill Bryson's new book" even if it made you laugh out loud. "Absolute" here is an intensifier. "Having a ball" means "Having a very good time;" "Having an absolute ball" means "having a very, very good time."

If someone says they're having a ball or having an absolute ball, it means they're having a great time. It's a positive statement unless it is said with a sarcastic tone.