Cookie Run: Kingdom Arena
Cookie Run: Kingdom's arena is a place where you can go to fight against other players of similar rank to you. You start at Chocolate II and slowly move up through the ranks to Master I as you defeat your opponents. As you grow to higher ranks, you also gain larger rewards both as a daily reward and at the end of the season.
Before getting into the arena, make sure you setup your attacking and defending teams. These may be the same for you but you can choose to have them be different. See below for where to set these up. The "Attackers" team is the one that you will use to attack other players with. The "Defending" team is the one that will defend against other players when they attack you.
To participate in the arena, you are presented with 5 opponents to fight. You can see their trophy count, team's power, and the units they are using on their team. You can choose between which ones you want to fight by pressing Battle next to the one you're interested in. Remember that higher Power is just generalized, so you may be able to take on opponents with higher power than you. Likewise, you may also lose to opponents with lower power than you.
At the bottom right of the list, there's a Refresh button. This button is incredibly important because if you're face with people you couldn't possibly beat, this button will refresh the list of opponents. You can only refresh every so often without spending Gems, but you'll be using this button a lot to move through the ranks. Refresh as often as possible if there are no opponents for you to take on.
Finally there's the different tabs for the arena. These let you see the people you're able to fight, the people who have fought you (you can take revenge on people who beat you if you're confident enough to fight back), the current overall ranking for the arena, and the current reward scheme for the current season.
By participating in the arena, you can accrue massive rewards over time. The arena can be a rather large source of gems in the game, especially as you get to higher ranks. You will also slowly gain medals that can be spent in the Medal Shop for a variety of different rewards, such as Soulshards or items to build in your Kingdom with.