Let's say you want your Garchomp as fast as possible you give him 31 IV's and 252 EV's in speed right? His nature gives him an extra 10% in the stat that it increases. However it also lowers one stat. So for Garchomp it will be SpA cause he doesn't use any SpA moves. So Garchomp will get a Jolly nature.
A list of natures and what they do can be found here:
https://bulbapedia.bulbagarden.net/wiki/Nature
If you give an Everstone to a valid breed partner you can pass it's nature to the offspring.
So let's say the female is a Garchomp with an Adamant nature but you want the offspring to have a Jolly nature you either breed him with a Ditto that has a Jolly nature or you find him a male partner that has a Jolly nature like Charizard.
In the pokedex from Serebii you can find the egg group of that pokémon and if you click it you can see the available species. Also if you scroll to the bottom you can see what egg moves it can learn if you click on the details you can find out what pokémon learns that move and passes it down to the offspring.
https://www.serebii.net/pokedex-xy/
Also if you want to pass IV's down you can give the other pokémon a Destiny Knot so IV's are passed down.