What is the meaning of nature "PKRS"?

Solution 1:

I think what you're referring to is "Pokerus", and it has a purple "PKRS" next to the nature of the pokemon.

Pokerus causes the infected Pokemon to gain twice as many "Effort Values" (EVs) as normal, even after the duration of the infection.

This causes it to become stronger more quickly than other pokemon.

Pokerus can't be cured in any way, and any one pokemon can only have pokerus once, after which it will be immune.

Pokerus is a transmittable disease, which can be transmitted to any other pokemon that has not had the disease before, and is directly next to a pokemon that currently has the disease in the party.

Solution 2:

You mean PKRS? it's a "virus" of sorts, it spreads to the rest of your party gradually but if you leave the pokemon with it in your party for too long, the effect stays but that pokemon can no longer spread it. The effect of it is double Effort Value gain, e.g. defeating a pokemon that gives one speed EV will give two. This stacks with all the other EV item (like the power bracer), so you can do EV training really fast.