What does it mean?

Hit me up when you're free.

Does it mean;
Call me when you're free.
or
Let me know when you're free.

Question

What does it mean?

Hit me up when you're free.

Does it mean;
Call me when you're free.
or
Let me know when you're free.

Answer

both!

it simply means to get into contact with the person when you are free, by any suitable method (call, text, messenger apps etc)

Answer

I would understand what you meant. 
To get in touch with someone. 
To contact someone.
In my humble opinion, it is not something people over 40 generally say.

Answer

It means to get in touch or contact me when you have time. It isn't specifically call or texting, it could either