Fuel is a mechanic that appears in Yoshi's Fruit Cart minigame in Nintendo Land. While the Yoshi Cart moves, the fuel slowly drains. When Yoshi Cart runs out of fuel, the Game Over screen appears. The amount of fuel left can be seen at the bottom of the screen.
Tip screen description
"The Yoshi Cart starts with just about enough fuel to travel all the way around the edge of the field once."