A British thermal unit, or BTU, is a unit of heat energy. One BTU is the amount of heat it takes to raise the temperature of one pound of water by one degree Fahrenheit.
The BTU is a way to measure energy in terms of heat. Since fuels exist to release energy when they burn, the BTU gives a plain yardstick for how much energy a given fuel holds, no matter what kind of fuel it is.
That makes it useful for comparing fuels head to head. A gallon of diesel carries more BTUs than a gallon of gasoline, and gasoline carries more than a gallon of ethanol. So a fuel with more BTUs per gallon does more work per gallon, which is exactly the comparison behind ideas like the gasoline gallon equivalent, where fuels are matched up by their energy content.
For anyone selling or buying fuel, the BTU is the honest measure under the surface. Two fuels can sell for the same price per gallon and still deliver very different energy, and counting the BTUs is how you see which one actually gives more for the money.
In useComparing the two fuels by BTUs, the buyer sees that a gallon of the cheaper one carries less energy, so the lower price does not mean a better deal per mile.
See also Gasoline gallon equivalent (GGE), API gravity