they are cards you get when you buy Mountain Dew and Doritos products. under each cap there is a code you enter the code at DewXP.com to get Double Xp the amount of time is different for each type of product.
Ex. 15 mins for a 20oz bottle of Mtn. Dew.
Ex 2. 90 for every Family Size bag of doritos

every code you enter also gets you PRIZE ENTRIES toward prizes like a hoodie, limited Edition Turtle Beaches, MW3 Xbox, a Jeep,

each code also gives you a Combat Card.
1 code = 1 combat card
there is 15 cards total.
in rows of 3 (5 cards in each row) 3 diff rows one for Mtn. Dew one for Doritos, and one for MW3

if you complete on row you get a free 15 mins of double xp
2 rows you get another 15 min so there 30mins
if you complete the 3rd row you get 15 more mins
once all of them are done you get 90mins of double xp

hoped this helped