Normally, a person enlists in the military for 2, 3, 4, or 6 years. That's the period of active duty in the contract. However, the person is actually obligated for a minimum of 8 years. The difference between active service and and the full term of the contract is reserve status, normally as "inactive ready reserve."

Military retirees are also placed into the inactive ready reserve status for many years. Even though I didn't retire out of the Air Force in 1992, I'm designated as a member of the inactive reserves until 2018. It's unlikely they'll reactive me, especially now that I've been out of uniform for 10 years, but it's still possible.