'Hit it off" means they made good friends with each other right away. They liked each other when they first met. It's an idiom that doesn't make sense to foreigners, but it means they "got along great" from the start.

It means that you found someone that you enjoy spending time with and that person seems to enjoy you. It suggests you will date again.

If you have hit it off with someone you are very lucky. It means you have found someone who you get on with well and have things in common with , it could be a new friend or a boyfriend/girlfriend.