Fuel can either mean the item fuel or a ship's fuel.

While applying thrust, the fuel in the Player's ship is slowly depleted. Once the fuel in a ship runs out, the ship travels at speed 10, as if running on fumes.

A ship's fuel can be replenished by using any purchased fuel items stored in the ship's cargo bay. Fuel can be purchased from Fuel Space Stations. Purchased fuel takes up 2 units of a ship's cargo space. The Fuel Station Alpha in The Cradle (Sector 0x0), sells fuel for $50.

Fuel can be crafted by combining 10 Aluminum Ore + 10 Ice Ore 

Fuel is used by clicking the "FUEL" button in your menu when it becomes RED.