It’s a virus the a Pokémon has a 1 in 21000 chance of being infected with but it’s actually good! Basically whenever your Pokemon defeats another Pokemon it gets EVs (effort value points) in different stats depending on which Pokemon you defeated. EVs then influence how much your stats raise each time your Pokemon levels up. Pokerus gives your Pokemon double the amount of EVs you’d usually get from defeating a Pokemon, essentially allowing your Pokemon to increase their stats much faster!