One of the many areas of the game is the arena. It becomes unlocked after purchasing a pass from the shop. Here you can use one of your idle adventurers to battle against a against an enemy of your choice while spectators watch.
There are 4 matches available at time. Beating one allows access to another, harder match. Each one gives a different reward. The harder the match, the better the reward. Rewards and match setups change every few days.
The easier matches give a little bit of money, the next hardest gives a rare crafting materials, the third hardest gives a piece of arena armor, and the last will give you either give you access to the Fishing Pier, or if you have that already, an arena weapon. These weapons and armor a little bit better than most of equipment your adventurers will find in dungeons, but getting them is far more time consuming and there's no guarantee the one you want will be available.
The matches themselves are a game of rock paper scissors. If you win, you do damage equal to your selected adventurer's damage. If you lose, you take damage based on your adventurer's defense. The battles goes on until one side's health reaches zero.
There are three types of matches: singles, doubles and triples.
The singles are the easier version, while the doubles and especially triples are quite a bit harder.
And of course, here's a (silent) video of a single and triple match.
Keep in mind this is only one area of the game. If you want information about the others areas, you can read a (little bit outdated now) news post I made about them: Indiedb.com