Where I am (East coast of the US,) a 'rap session' is most commonly used as a musical term. Where one person, or a group of people, practice their freestyle rapping. It can be casual or competitive depending on the setting. An example would be in the movie "8 Mile," in the where Eminem and his friends meet and take turns free-styling. (That scene might be wrong. It's been a long time since I've seen that movie, but it's more or less an accurate depiction of what I define a 'rap session' as.)

People collect to talk to each other. I never use this term. No examples, sorry.

It's an American term - only very rarely would you hear it where I am. A rap session is when you get together with some others and have an open and informal discussion on some common interest.