Coupon shards are awarded by completing daily tasks. The shards can then be exchanged for a coupon. It costs 6 coupon shards to get a coupon.
On February 8th 2017, in a run of 300 coupons the ratio was: 0,67% 5star, 6% 4star, 52% 3star, 24% 2star, 8,33% 3star essence and 9% 2star essence.
Read more about the various currencies.