Moistness is an in-game mechanic related to Oxygen and having to do with water. Moistness is an indicator of how long a animal can survive on land before they begin to take damage.
Terrestrial Animals/Aerial Animals
Terrestrial/Aerial animals have no moistness since they live on land. They instead use oxygen.
Fully Aquatic Animals
Fully Aquatic Animals have a moistness meter of 30, meaning once that reaches 0 they take damage while still on land or above the surface of the water. Most Fully Aquatic Animals have oxygen as well, such as Mosasaurus.
Hybrid dinosaurs have no moistness, but they do have increased oxygen and the ability to swim with no major speed loss.
At first many semi aquatic creatures also had a moistness bar, such as Sarcosuchus, but this has been removed from them.
The Megalodon has the most moistness of any animal in the game, with 150 moistness.