Alright, let's see if I can do this:

Every Pokemon has magical hidden numbers. You can't see them, but everyone has a counter for them. They're called EV's, or Effort Values. If handled correctly, they'll allow a Pokemon to reach their maximum potential. There's a separate EV counter for every stat; so HP EV's are calculated separately from Attack EV's, etc.

Now, let's say that, with 0 EV's, a Pokemon's HP at level 100 is 350. Okay? For every 4 EV's you get, their stat at level 100 will increase by one. So that Pokemon with 4 HP EV's will now have an HP of 351. So on and so forth. Unfortunately, you cannot just raise stats to 999; EV's have a limit of 255 EV's per stat, and 510 EV's overall. So you can have 102 EV's in 5 stats, or 255 in 2 stats, as long as the total does not exceed 510 (you won't be penalized for it, because you can't actually surpass 510). As a sidenote, as you can imagine, the best EV-spread for maxing out two stats in 252 EV's in two stats, leaving you with 6 in another. Why? 255 is not divisible by 4, meaning you have 3 wasted EV's in that stat that could've gone to something else.

Now, how do you acquire these little numbers? By fighting Pokemon. Every Pokemon defeated gives EV's. For a full list of what Pokemon drop what EV's, check this page on Psypoke. Remember, you'll have to keep track of the EV's gained yourself. Also note that EV's can be lost (in case you accidentally got an EV you didn't want) by feeding the Pokemon a berry that says "Raises happiness but lowers base [whatever]". This will decrease the EV count by 10.

So, hope that made sense.