What does invite code from a friend mean?

I'm curious to know, when someone mentions an "invite code from a friend," what exactly does that entail? Is it a special code that gives access to exclusive features or discounts? Does it serve as a referral system where both the friend and the person invited benefit in some way? Or is it simply a way for a friend to introduce you to a new platform or service? I'd appreciate any clarification on this topic.