So you are asking yourself what is 'canopy management' and why would you need to manage the canopy of grape vines. A simple answer is to expose the vine (i.e. the grape bunches in the growing zone) to as much appropriate (direct and indirect) sunlight as possible and to give each cluster, bunch of grapes the best possible chance of achieving full physiological ripeness, aroma, flavour and varietal character development.
A complex series of techniques including: vine spacing (starting with planting distances), trellising, shoot positioning, and leaf removal to improve both light and air circulation, to have the affect of creating the optimal grape-growing environment for maximum colour, aromas, flavour and ripeness of the grapes.

 

Such techniques are and should be very specific to each vineyard site, contingent on such things as soil fertility, grape varietal, the age of the vine, unique micro-climate and seasonal influences. Proper canopy management can affect the colour, flavour, and/or structure of grapes. It can also help prevent disease problems.
For instance, the careful removing of select leaves and shoots improves aeration, thereby reducing susceptibility to excess moisture inside the canopy - leading to rot and mildew on grape bunches. There is however a fine and careful art to removing excess foliage and it can be different with each new growing season.

Enough healthy leaves must be left on the vine to provide the required energy for grape maturation (i.e. maximum sunlight interception and optimum photosynthesis) - excessive leaf removal can bleach the fruit's colour, cause sunburn on the surface of the grapes skin, or even prevent ripening of the bunches.
On the other hand, vineyards growing in warmer, drier areas require less leaf removal than those in cooler, moist climates. In the end, good canopy management - with the result of the perfect balance between vine growth and grape production - can mean the difference between an ordinary wine and one of balanced quality.