You use a wildcard to be able to use things such as three attachments, two grenades, and two perks from the same tier. It does however take up one of your 10 slots on your class.

soberecon wrote can somebody explain to me what the "wildcard" thing is iv'e hard about it and i am really confused on it

Wildcards basically let you bend the rules a bit.
For example, there is a Wildcard called Perk 1 Greed. You spend a point on this Wildcard and it will allow you to have another Perk 1. So I could say have both Hardline and Ghost. Adding a perk this way will cost two points in total, one for the Wildcard (Perk 1 Greed) and another for the second perk (Ghost). There are other wildcards that allow you to have two primary weapons, 3 attachments on a gun, etc. Really neat concept.