Crown reduction refers to the practice of reducing the height and/or spread of the crown (upper foliage area) of the tree. It is used for a number of reasons. For example, to reduce stress on the tree, or due to site requirements such as limited space. The main framework of the crown should be retained during pruning, and the final result will simply outline a smaller similar shaped crown.
(Please refer to diagram below. The lighter green area shows the sections to be pruned)