The Fuel Canister is a portable container for transporting Petrol/Gasoline from Kali-Gas to where it is needed. A 20 Liter canister can be found in the garden shed outside the Player's Home.
To fill the canister, take it to Kali-Gas and unscrew the yellow spout with an empty hand (Left click to tighten; right click to loosen) until it's fully removed. You're then free to fill the canister from the fuel pump as much or as little as you please. Once finished, tighten the yellow spout back onto the canister, and use it to fuel whichever machine you need.
Using the fuel canister is the only way to refuel the Generator, but it can fill the tank of any machine: The Konig, the Wood Splitter, etc.
A second fuel canister can be found inside the Cottage once the player has purchased it, but it is only a 10 Liter variant.