The canopy is the "roof" a woodland or forest, the more or less continuous uppermost layer of vegetation formed by the crown (foliage and branches) of the tallest trees or the upper foliage and branches of an individual tree or shrub.
The density of the canopy is a parameter used to differentiate woodland from a forest.  A forest form a largely-closed canopy, in other words, the branches and foliage of trees interlock overhead, cover and furnish nearly a continuous shade, while a woodland has a largely-open canopy (the tree crowns cover less than 20-30 % of the ground) with sunlight penetrating between trees.