CDR was a stat that reduced the cooldown of your champion's abilities or the amount of time before an ability can be used again after activation. This was determined by a percentage and was granted by items like Luden's Echo, buffs like the Blue Sentinel's Crest of Insight, and runes like Transcendence.

How does the CDR Rune work?

Metroid Dread - The Loop

Cooldown Reduction (abbreviated as CDR) is a champion statistic that either reduces the cooldown of champion abilities, or the amount of time before an ability can be used again after activation, by a percentage. This stat is tracked and can be seen in the champion stats tooltip.

How much CDR is 10 a haste?

In other words the first 10 Ability Haste give you approximately 9.1% CDR, the next 10 Haste give you only an additional 7.6% CDR and the ones after that give you only an extra 6.4% CDR.