I don't get it

Gyms are places where you can battle other teams to take over their spots, or leave your Pokémon to earn PokéCoins. You can also retrieve items and battle powerful Pokémon in Raid Battles. If you have any questions, feel free to ask me anytime, Hou!

If you win a battle and capture a gym, you can get up to 50 coins per day depending on how long you hold it. If there's an open spot in a gym your team controls, it's a good idea to leave a Pokémon there. Also, there's a bonus to the items you get from gyms your team controls.

If you put it in a gym, it takes about 8 hours to reach the maximum of 50 coins. However, it doesn't accumulate, so if you try to defend it for 2 days, you'll only get 50 coins when you return.