Simple
Doubles the Pokémon's stat modifiers. These doubled modifiers are still capped at -6 or 6 stages.
Full effect
Each stage of this Pokémon's stat modifiers acts as two stages. These doubled stages are still limited to a minimum of -6 and a maximum of 6. This Pokémon can still accumulate less than -3 or more than 3 stages of stat modifiers, even though the extra ones have no effect after doubling.
In game description
Doubles the effects of the Pokémon's stat changes.
Pokemon with Simple
Using Simple in Cobblemon
Abilities are passive and always active, so Simple costs you nothing to run. If it is marked hidden on a species above, expect it to be harder to obtain: servers differ in whether hidden abilities come from breeding, an Ability Patch, or events.
See the Pokedex for which abilities each species can have, and IVs, EVs and breeding for how to actually pass one down.