The joke is that trainers are concerned with the IVs of their Pokémon, which can be passed down to other Pokémon via breeding (putting two Pokémon in the day care leaves a chance of them having an egg). IVs effect the stats of Pokémon, whether they have good or bad stats (best is Base 15, worst is base 0, most Pokémon will probably roll a 7-12. Rolling a 15 for every IV is super rare and super good for competitive Pokémon).
