The "mind projection fallacy" occurs when somebody expects an overly direct resemblance between the intuitive language of the mind, and the language of physical reality.
As an archetypal example: Suppose you flip a coin, slap it down over your wrist, and don't yet look at it. Does it make sense to say that the probability of the coin being 'heads' is 1/2? How can this be true, when the coin itself is either objectively heads or objectively tails?
To which we might reply: Uncertainty is in the mind, not in reality. If you're ignorant about a coin, that's not a fact about the coin, it's a fact about you. A blank map does not correspond to a blank territory: if you come to a part of the country where the map is blank because nobody's mapped the country yet, you won't actually see a vast white space stretching out in front of you.
It makes sense that your brain, the map, has an internal measure of how it's more or less sure of something. But that doesn't mean the coin itself has to contain a corresponding quantity of increased or decreased sureness; it can just be heads or tails.
"Ontology" refers to the elementary or basic components of a system. The ontology of your map includes an intuitive measure of your uncertainty - we can imagine it as something your brain represents as a primitive type in its computations, like 'floating-point numbers' are a primitive type in CPUs. The Mind Projection Fallacy is when we reason as if the territory, the physical universe and its laws, must have the same sort of ontology as the map.
See also: