As the name suggests, this whisky is produced by blending any number of malt and/ or grain whiskies from multiple distilleries. It is up to the master blender to use their skill to produce the specified flavour profile.
Blending whisky allows for a far more consistent outcome then say, a single cask whisky. As you can imagine, variations and inconsistencies can be masked or hidden by combining hundreds, sometimes thousands of casks together. The second benefit is that grain whisky can be used in the blend to help moderate the price whilst still achieving a high quality product.