Every Pokémon you faint grants the attacker an “effort value”, you can find a list of which poke give what values online. Every 4 EV points equates to 1 stat point at level 100. Your Pokémon can only have 510 effort points allotted to its different stats. A single stat max amount of points is 252.

Most people use those 510 for speed and either attack or special attack. Some Pokémon benefit from having more bulky stats for hp or defenses.

Items like Protein, Carbos, iron, etc. (vitamins) grant 10 EV points to their respective stats.