Yes I can explain although this is only my understanding I am not a professor.
There is a very strong gesture someone will be "in your face". For example during a professional sports game (commonly in hockey or basketball) one player may get very close to another player. Face to Face, and taunt them about their performance as an intimidation tactic even at times yelling while just a few centimeters away. We would describe this saying "got in his face".
The purpose is to make the other person uncomfortable by violating their personal boundary. Its a complicated gesture which can have many meanings.
It has been adapted in a more friendly way, typically used when friends are competing at something. For example if I am playing a video game with a friend and I win I might say "oohhhh In your FACE!" but usually laughing and smiling at the same time. In that use it can be a friendly. I would typically only use that with close male friends or with a woman only your girlfriend but she might get revenge later ;)
Thanks. I hear this so often if I watch the Simpsons. Homer says this occasionaly. It is very funny and I know when to say to who, but I wasn't sure where this was originated.
In the past few years, "in your face" has been broadly adapted as well. Example from just a few days ago, "in your face politics" as the election went full force.