The phrase "hang in there" means to not give up on something, even if it's difficult. In other words, it means "Don't give up!"
You can tell someone this to encourage them:
Hang in there. You'll start to get better at it pretty soon.
Hang in there. It's almost over.
You can also ask someone if they're "hanging in there". This means, "Are you OK?" or "Are you going to be able to do it? You can ask this when someone's doing something difficult or painful.
In casual spoken English, you can leave off the "g" sound at the end of many "-ing" words. So instead of "hanging", you can say "hangin'".