What does the term "Rollover" minute mean?

Rollover minutes are offered with AT&T calling plans.  They accumulate when you select a calling plan and do not use all of the anytime minutes.  (i.e. You sign up for the 450 minute plan and only use 400 minutes.  50 minutes rollover to the next month and can be used within a year.) The specific definition and details of how their rollover minutes work is included below.

Rollover® Minutes accumulate and expire through 12 rolling bill periods. Bill Period 1 (activation) unused Anytime Minutes will not carry over. Bill Period 2 unused Anytime Minutes will begin to carry over. Rollover® Minutes accumulated starting with Bill Period 2 will expire each bill period as they reach a 12 bill period age. Rollover® Minutes will also expire immediately upon default or if customer changes to a non-Rollover® plan. If you change plans (including the formation of a FamilyTalk plan), or if an existing subscriber joins your existing FamilyTalk plan, any accumulated Rollover® Minutes in excess of your new plan or the primary FamilyTalk line's included Anytime Minutes will expire. Rollover® Minutes are not redeemable for cash or credit and are not transferable.