Wow. You guys really are new to this. No Big Deal. There are fewer rules in soccer than in other sports.
Any infraction in one's own Penalty Area will result in a Penalty Kick by the opposition from the "spot" 12 yards from goal. PK is the vernacular. Our guy kicked their guy in the face, while both were in the Penalty Area. That is (understandably) a foul. Colin Biros did not connect much, but it was enough. I have seen exponentially much worse, some really gruesome stuff. And that kind of stuff is what we all want to avoid. Ref could have let this one go, but he didn't. By the letter of the law, it was the correct call for the Ref to make.
BTW, dimensions on Penalty Area, Goal Area, Center Circle and actual Goal are some of the only things that have rigorously standardized dimensions. Field length and width have a general range. Wikpedia is all over this one.
https://en.wikipedia.org/wiki/Penalty_area