The Fuel Level Sender is part of the car electrics system. We have 2 associated problem symptoms connected to it.
The fuel gauge doesn't move regardless of the amount of fuel in the tank, it's stuck in the same position and it's impossible to tell how much fuel is actually in the tank.
The fuel gauge is not where you'd expect it to be after putting fuel in the tank. After completely filling the tank the fuel gauge does not read full. When the tank is empty the gauge reads that it has fuel.