This is when the 'crown' of the tree is made smaller; either all over, or in a certain area to take care of an imbalance in the shape. A good reduction will leave the tree with a strong framework of large branches which then support the smaller branches and twigs (leaf bearing).
When would you have a reduction done?
When a tree has become too large for its position, but removal is undesirable.
To address an imbalance, or pruning to clear an object (e.g. building street lights).
To maintain a specific shape.
When should I NOT have a reduction?
Certain species react poorly to reduction and others should only be reduced at certain times of the year.
When a tree is in the wrong position, or the space is not fit for the tree, it may be better to remove it and plant a more suitable tree.