This is called bedding sand or concrete sand. It is a coarse, angular sand that has been washed to remove dirt and clay. This fills the paver joints from the bottom up and also allows for removal of minor height differences between pavers during the first compaction. It should not be used to fill large depressions in the base material. Bedding sand is typically installed at a one-inch depth and should be loosely and evenly screeded. Be careful to cause no compaction of the sand before setting the pavers. Screed sand and lay pavers in small sections at a time.
What is the sand on top of the base material for?