Spent a few minutes researching because I was almost sure it was just measuring resistance but that is different depending on materials.

They measure moisture by using a scale of wood equivalent moisture. Basically no matter what you stick it in, the pins measure resistance across the probes and give a number as an equivalent reading for sticking them in wood that is wet. Apparently wood is pretty consistent about resistance regardless ofof what type of wood it is. And since American houses are primarily made of wood, we only care about how wet the wood in a home could be.