An importer is a company that brings fuel into a state, or into the country, from outside it. Because that fuel was not taxed where it crossed in, the importer owes the tax on it.
Fuel does not always come from a refinery or terminal inside the place where it is sold. Sometimes a company brings it across a border, from another country or from another state. The company that does that is the importer of the fuel.
The tax rules single out the importer for a reason. Fuel taxes are owed where the fuel is used or sold, and imported fuel may arrive without that tax already paid. So the rules name the importer as the responsible party, the one who has to report the fuel and pay the tax that did not get collected upstream.
For an operator, this is why crossing a state line with fuel is not a casual act. Bringing in product to sell can make a company an importer with tax to file, even when it feels like ordinary buying. Knowing where fuel comes in, and who is on the hook when it does, keeps a company clear of a tax bill it did not see coming.
In useA marketer that trucks fuel in from a terminal across the state line is the importer of that fuel, and it owes its new state the tax that was never paid on the way in.
See also Motor fuel excise tax, Position holder, Blender