the understory varies a lot from forest to forest- but every where, it is darker, there is less wind, and it is more humid that the canopy above it. The trees are either seedlings struggling to survive, or are older trees that have been suppressed by the lack of sunlight.
There are different ways that plants have adapted to live within the forest in the understory, competing for sunlight and nutrients. Some grow in the spring or fall when the canopy is not completely covered by leaves. Others have adapted to live with very little, or no direct sunlight.
many birds and animals spend most of their lives in the understory.