A rainforest has many tall trees, often with large canopies and vines growing. As a result, oftentimes at ground level there are many shadows and darker shaded areas. An adaption to this environment might be dark fur to blend in with the background, so that prey can hide from predators or a predator can sneak up on prey. This wouldn't work as well on the tundra, where there is plenty of light, few trees, and the primary colors of the landscape are the green/yellow of the grasses.
Another example might be the ability to swing from tree to tree as you see with animals like spider monkeys. There are few trees on the tundra so this wouldn't be a useful adaption, but in a rainforest where there are plenty of trees clustered close together this is very useful.