Class advantage will give you a 50% damage boost against certain enemies and reduce their damage against you by 50%. In general, Carnivores are greater than Herbivores, Herbivores are greater than Pterosaurs, Pterosaurs are greater than Amphibians, and Amphibians are greater than Carnivores.
Aquatic Class Advantage:
Much like the relationship of classes for dinosaurs, certain classes of aquatic creatures have their own advantage over others. In general, Surface creatures are greater than Cave creatures, Cave creatures are greater than Reef creatures, and Reef creatures are greater than Surface creatures.
Cenozoic Class Advantage:
Similar to the Aquatic and Jurassic battles, Cenozoic creatures also have their own strengths and weaknesses. Savannah creatures are stronger against Cavern creatures, Cavern creatures are stronger against Snow creatures, and Snow creatures are stronger against Savannah creatures.
You know in pokemon fire (red) beats grass (green) which beats electric (yellow) which beats water (blue) which beats fire? Its the same in jurassic world the game, where carnivores [red] beat herbivores [green] which beat flyers [yellow] which beat aquatics [blue] which beat carnivores? Its the same pattern!
The Gaming Beaver also stated this in one of his vids. The one called "Tapejara vs Koolasuchus".