Beli is the currency used in Blox Fruits. It can be gained from killing NPCs, completing Quests, collecting Chests, doing Factory Raids, and fighting Raid Bosses. It can be used to buy Weapons, Blox Fruits or Abilities. You can also get beli by completing bounties on other players or by killing youtubers in good standings.
Beli is also obtainable using Robux, but it's not recommended because you can get Beli very easily. Getting the 2x Beli gamepass will help you earn cash a lot quicker, so it's recommended for you to get it if possible.
You can farm beli easily by farming bosses.
The code Fudd10 gives you 1 beli, sadly. (its a troll code)