What does perfect binding mean in bookbinding?

A perfect bound publication has all sides of the pages trimmed even at the spine. This called the "grind". The pages are "ground-off" to insure that the inside pages' edges are all even.

Perfect binding glues all of the pages to the spine of the cover. The result is a completely even edge on the outside. This binding is optimal for those with many pages and a heavier paper weight cover. It is commonly used for higher volume magazines and luxury catalogs.

Resource : heidelprint.com