There are 2 indicators of fuel you can see. You have your reservoir and your main fuel tank. Your reservoir has a capacity of exactly 1t of fuel. That is that little bar on top of the bigger bar on your lower right HUD, the bigger bar showing your main fuel tank. During normal flight or super-cruise, that tank (the reservoir) gets depleted. Once empty, it will automatically refill another 1t fuel from your main fuel tank. So let's say you fly a ship with 32t of fuel capacity. Frame Shift Drives have a max fuel limit which they can drain while performing jumps. So jumping your maximum range of let's say 20LY would drain your MAIN fuel tank by (let's make an example again) 4t of fuel, leaving you on 28t of fuel to cruise or jump with. This is basically everything about the fuel system in the game.
Oh and also: buy a fuel scoop. trust me, you won't regret it, especially in bigger ships