"Hitting on" someone means trying to create a sexual or romantic situation with them. Here are some ways to "hit on" someone:

• asking them out on a date
• asking for their phone number or email address
• complimenting their appearance

You can't really "hit on" someone who you're already in a relationship with. People usually "hit on" people who they've just recently met.

The phrase "hit on" doesn't sound especially negative. You can say that someone "hit on" you, whether you liked it or disliked it.