Crown thinning is the selective removing of stems and branches, to increase light penetration and air movement ,throughout the crown of a tree.
When to crown thin a tree
To allow more light through the crown by reducing the density.
To allow the wind to penetrate through the crown, therefore reducing the likelihood of damage to the tree or its environment.
When NOT to crown thin a tree
Crown thinning certain species can cause the tree to produce more growth along the branches, therefore undoing the works you've undertaken