It could be the low fuel level warning light, which usually comes on when you are down to the last few litres. Maybe there is a fault with the sensor, or its circuit.
It could be a number of things.. Make and model could help.
Have you ran the tank dry at any point. . Is the fuel cap secure. It could be a fuel sensor or a sender unit fault.
I never let the fuel tank get really low and so I've never seen a warning light there. My daughter filled it up for me a few weeks ago and I've hardly used it since so I know it isn't low on fuel. Dave, it's a Kia Venga.
Have researched it on the Kia Forum and it would seem that if the car was left running while it was filled it up, this warning light may come on or flash.
It often rectifies itself as the car gets run.
It's nothing serious to worry about.