Its a slang, which means: To give someone a phone call, page, instant message, email, any form of communication. For example: Just hit me up later. Hit me up on my celly, I'll pick up right away.

"hit (someone) up" is a famous American idiom. It means to ask for a loan or a gift. It's not very polite to hit someone up unless the person is family or a best friend. Regarding asking someone to text or call you is a lot different from asking them for money or some other valuable. I read the entry at www.urbandictionary.com/ This meaning is new to me; so is "celly" to mean cell phone. Slang is using an existing word (or expression) to mean something else and used by only one group of people. It seems "hit me up" used to be an idiom which is now slang with a new meaning which focuses on cell phone calling or texting. "hit" I have heard people say when calling, but no "up". "hit" = call. Can be a cell phone, can be a regular one.

I agree with deep_blue. If you're speaking with a native English speaker and you said "Hit me up later," they'd most likely understand what you meant.

Stick to give me a call or send me a text. Or msg me later

Rather cool but I'm afraid that not everyone can understand it. http://www.urbandictionary.com/define.php?term=hit-me-up