I dont PVP much either to be honest, these are mostly so i can DESTROY the Battle Tree =3=
(which im close to being able to start doing, this Salamance im working on is step one...)
well pokemon can have a total of 510 EV's (or Effort Values)
4 EV = 1 point in a given stat.
you can have 252 EV's in a single stat maximum (within the constraints of having 510 EV's total)
Meaning 252 divided by 4 = 63 points in 2 stats of your choice (Plus one stat left over to go anywhere)
I'll use my Salamance as an example, i have 252 EV's in both Attack and Speed and threw the spare one on his HP.
Meaning he'll have 63 more points in each stat (by lvl 100).
Now usually when you play the game without bothering with that EV's are usually equally spread throughout your pokemon.
you get EV's by defeating pokemon, using the bouncy house thing, or sending them to the training Island in the Pelago.
Fastest way is by getting the power items and chaining certain pokemon.
For Example i want physical attack, so i go chain a bunch of Pikipek or Grubbin or something that gives Att ev's.
You can look at you EV distributions anytime by going to Summary and Pressing Y
It's just a way to further improve your pokemans ;p