1. What is stamina?
Stamina is required for getting experience from monsters. Without stamina you can still kill and drop items from monsters, but you don't get any experience points.
2. Stamina reset.
Every day on 00:00 (UTC +1) the stamina is reset back to 6 hours. If you have't used all your stamina from the day before it will go to your Stamina Bank, which has capacity of 16 hours. You have to login at least once during the day to have your stamina transfer to bank when reset occurs.
3. Stamina bank and refiller.
Your bank has a maximum capacity of 16 hours of stamina and you can withdraw up to 4 hours of stamina from your bank with each Stamina Refiller. You can buy Stamina Refiller from cash shop or get them in-game from rare drops.
4. Commands
!stamina - You can check your remaining stamina as well as how much stamina you have in your bank.
!stamina on/off - you can temporarily turn your stamina off, so you don't waste it when you are pvp fighting or collecting items from weak monsters. There is a 3 minutes cooldown between uses.