Originally, someone who was a winner of the game called "pogs", also known as milk caps. Used ironically in the video that coined the term.
Later, the name of a certain twitch emote used to express hype/excitement, usually when the streamer does something surprisingly good. The twitch emote was the head of the person who coined the term pogchamp, while he made a specific facial expression that looked hyped/excited.
In this context, it's used as a term of endearment. It has no immediate relation to any of its preceding meanings; part of the humor is that it leaves people who know about pogchamp guessing how it relates to those meanings, while it leads people who don't know about pogchamp down a rabbit hole of twitch culture.
It could means someone who's often hyped/excited. It could mean someone who rouses hype/excitement. It could be someone who wins at children's games, alluding to them being a child and thus being similar to terms of endearment like "baby".