You will receive six (6) empty MAC pigment jars (7.5g) for MAC's Back to M·A·C Program.
From MAC website:
"Because we share your commitment to the environment, M·A·C accepts returns of its primary packaging through the Back to M·A·C Program. By returning six  M·A·C primary packaging containers to a M·A·C counter or M·A·C Cosmetics online, you’ll receive a free M·A·C lipstick of your choice as our thanks to you.
Please note that some MAC counters may allow you to redeem the empty containers for an eye shadow as well (limited edition products are usually excluded from promotion). All pigment jars are guaranteed to be authentic. Pigment jars may or may not come with original boxes.