Fuel is the propellant for the rocketships.
The Martian fuel is produced via the Sabatier reaction, breaking down water from ice into oxygen and hydrogen via electrolysis, then combining hydrogen with carbon dioxide from the atmosphere to create small amounts of water and methane fuel. The amount of fuel (and thrust) needed to escape the Martian orbit is much lower than needed to escape Earth, due to Mars being smaller (only having 0.375 times the gravitational pull of Earth), and having a much thinner atmosphere.
Martia tokens can be staked to receive a share of the 1,500 Fuel tokens awarded each hour.
Fuel is not a direct gameplay input into any of the items in the colony, but is used for Colony Development.