Perks add extra abilities to the player (such as increased damage output, enhanced minimap, or enemy debuffs). If the right perks are used together, it can create powerful tactical advantages.
Using a perk in a match gives it experience. After the same perk has been use in a sufficient amount of matches, it will reach level 2. At level 2 a new ability is added to the perks, as well the the previously existing one, essentially giving the player one perk that does two things.
There are 15 perks, and 3 perk slots per loadout. The first two slots are unlocked at level 1, the last is only unlocked at level 45. Only two perks are available for use when first starting multiplayer. As the player increases in level, more and more perks become available.