In an arena there is a certain time control/variant, and there is a timer for 30 minutes, 1 hour, or 2 hours. You win as many games as you can within that time period; when you finish a game, you click "Next Arena Game" and get immediately paired up with another person that just finished their game. You get 2 points for winning a game, and 1 point for drawing. When you start to go on a win streak, then you accumulate more points for each win. Here's a helpful article:
https://support.chess.com/article/335-what-are-arena-tournaments
Hope that helps!
Just started playing tournaments here on chess.com. I know what "round Robbin" and "Swiss" are from years of OTB play, but what is arena? How does it work?