The Atlas is a book that Spyro, more specifically, the player, can use. It shows how many dragon eggs you've found, gems found, Homeworlds and levels completed and unlocked. In general, it is a "check list," to see how far the player is until finishing the game.
Once you have completed the Sparx Level Bugbot Factory, Sparx will unlocks the ability to warp you into any level using the Atlas.
For Spyro: Year of the Dragon, there were plans for the Atlas to show details of all the baby dragons saved, which included their name, breed and favorite color.[1]
