Starting in Gen 5, most(but not all) Pokémon were given access to another ability, referred to as a Hidden Ability. It just gives the Pokemon another possible ability that's usually harder to get.
Explaining on how to get the HA's and how to get then is a bit long so I'll link you to a Bulbapedia article on Hidden Abilities.
Source: Experience and Bulbapedia
Hope I helped!