-You do this one thing for me and I'll leave you alone.
-I know what you're about, Phelan.
Lex Luthor told me. Meaning I know what you're aiming for?
I believe BE usage of this preposition is/was roughly that meaning. However, in recent years in AE the phrase has taken on a rather bizarre meaning as shorthand for what drives one, what one's belief system is, where one's interest lies.

"I'm all about conservation and recycling. I'm just saying that every once in a while, bottled water can be really convenient."

"John loves the new discount store. You know him, he's totally about saving money whenever possible."
Thank you, Delmobile!