That's OK.
"Tonne miles per gallon" is just a generic measure for the fuel efficiency of moving a vehicle (or anything come to that) from A to B. Broadly speaking the bigger and heavier the better the tonne.miles/gallon. You can go into all the engineering and physics, drag vs rolling resistance etc, but it's largely irrelevant if all you want to do is compare different ways of moving stuff about.
If you start to break it down into payload and gross mass then it gets more involved, but the measure can still be used as a comparator. If you want to go to one end of a spectrum you can say that a single driver in a car going to the supermarket for a bottle of milk is about as bad as you get if you only consider the milk as the payload. The measure is usually applied with full payloads since this is the most efficient, and empty truck is fairly wasteful.
It's useful to compare bulk carrying of freight, big trucks are better than small trucks, trains are better still and ships about the best current bulk transportation. Planes aren't good, but they're quick.
It's just another "metric", useful for some purposes but not all. For domestic cars I used to estimate using 50 tonne.miles per gallon as a very simple rule of thumb, but that's a bit out of date, it's probably nearer 60 tmpg these days. It's been steadily improving over the years. Hybrids etc have changed things substantially (energy recovery systems and so on).