Usually within context, it is an idiom that basically means 'flirting'. So if I am "hitting on" someone, I am flirting with them.

It means someone is being flirtatious towards someone else. It essentially means someone is attracted to another person and wants to pursue further contact.

Like if you are talking to a girl you are trying to become involved with. For example if you were talking to a girl you found attractive at a bar trying to get with her like to have sex or to become a girlfriend.