Usually means someone is being bothered or overwhelmed by how close another is to them. He got in my face during the argument, making me really uncomfortable.
Pro Tip 2/3
Emotional charge
The phrase often indicates that someone is being emotionally intense or confrontational. She was all in my face about my mistake, making me feel worse.
Pro Tip 3/3
Invading space
It's used to express a violation of someone's personal space or boundaries. The pushy salesman was right in my face, and I had to step back.