From publisher blurb:
Meant to replace the race feature of a D&D 5e character, this module allows players nearly infinite possible ways to create balanced homebrew races.
More than that, though, it allows players and DMs to have conversations about the mechanics of their game world. For example, in a campaign world that is all human, players can decide what physical and cultural attributes make their human distinct from all the others. Weapons training? Magic training? A beefy build? Mechanics usually gated behind non-human races are suddenly available to players in a restricted campaign setting. Everyone in the party can be human and still mechanically distinct.
Likewise, in a setting with no limits on race, this splat book allows players to let their imaginations take control of character creation. Cactus-folk? Done. Half-tarrasque, half-ochre jelly? Done.