The higher the moisture content, the lower the resistance. They come with different measurement indicators. Signal penetration for pinless meters is up to 1 inch for both hardwood and softwood. The meter can be moved across the surface to identify pockets of moisture in a wood block or plank. It is relatively unaffected by temperature.
Rough surfaces have very little effect on the reading.
Measurements can also be taken through coating, varnish or paint without damage to the surface. Deciding which kind of moisture meter to buy is a matter of sorting through the features you think you will need, and how often you may use it, and then understanding the various features and benefits. A clear analog or digital dial. For a probe meter, the ability to take external probes, and a selection of pin sizes. Just test for moisture at several locations in the room and average the results.