This feature is aimed at players who are only interested in perhaps a single class or race from one of the core rulebooks and allows you to purchase the rules for that, without having to purchase each of the rulebooks that contain those rules.
As an example, say you love playing Rogues and want to create one with the Swashbuckler subclass, this is in the Sword Coast Adventurer's Guide. Rather than purchase the whole of the Sword Coast Adventurer's Guide, you could instead just purchase the Class Options bundle that will unlock all the subclasses available in that book.
You might further decide that you want that character to be a Tabaxi, but that race is in Volo's Guide to Monsters. You can purchase just the Tabaxi race, unlocking all of the information, lore and rules for them.