Title text:
The arbiter gave my knight a red card for capturing with cleats up :(
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3268/
It’s a football (or soccer, if you’re so inclined) reference.
Overly simplified, the attacking side needs to have one other player besides the opposing goalkeeper in front of the ball to not risk being called off-side.
There is a lot more nuance to the rule, and even I don’t fully understand it 100%, but for the sake of this joke it should suffice.