It's just a metaphor. I think it would relate better to contacting someone. To hit someone up is just metaphorical slang for getting in touch. Hit me up if you want more help : )

What they both said is correct, but it is also used to tell somebody to get in touch with you later. Ex, A bro, hit me up after you finish your homework.

It means to ask for things from someone. It's very, very casual, and implies that the person is doing you a favor. You might hear someone say, "I'm really broke, I need to hit my Dad up for some cash", or "Do you think George would let me hit him up for the loan of his car next week?"

"To hit up" has a lot of meanings. One of them is "to ask for money". It's informal North American slang.