The process of scaling numbers so that they all add up to one is called normalization.
When you enable normalization for smooth skin, Maya normalizes weight values so that as you change one weight value, other points affected by the same influence are automatically changed.
When normalization is on, Maya considers the weights of all skin points affected by a particular influence as being relative to one another. Making sure that all affected skin point values continue to add up to one lets Maya keep track of their relative influence.
The Normalize Weights drop-down (found in the Smooth Bind Options, the Paint Skin Weights Tool, and in the skinCluster node) lets you set how you want Maya to normalize weights. See Set normalization mode and normalize weights and Normalize Weights mode.