I take it to mean it reduces the cooldown on which further item/rune/masteries CDR is calculated. To put some numbers with it, the new Q cooldown is 10 seconds.
At level 1 with 10% CDR, the cooldown is 9 sec (10 sec * 1.0 * 0.9).
At level 6 with 0% CDR, the cooldown is 8 sec (10 sec * 1.0 * 0.8).
At level 6 with 10% CDR, the cooldown is 7.2 sec (10 sec * 0.8 * 0.9) .
At level 11 with 30% CDR, the cooldown is 4.2 sec (10 sec * 0.6 * 0.7).
At level 16 with 30% CDR, the cooldown is 2.8 sec (10 sec * 0.4 * 0.7).
At level 16 with 45% CDR, the cooldown is 2.2 sec (10 sec * 0.4 * 0.55).