HMU is an abbreviation used in texting and Internet slang that stands for "hit me up." It is an idiomatic way of telling someone to contact the requester via phone or email.
The idiom "hit up" is used in vernacular spoken English to express that someone has made a request of something, usually a loan, an object of monetary value or a favor. For example, a beggar asking for spare change is said to be "hitting up" passersby for money. In the context of texting and Internet slang, HMU refers to asking someone else to contact them, and generally is not used to refer to asking others for money or other favors. It is another way of asking someone to call, text or email back.