What does Tenacity do?

Its CC resist.. like 25 Tenacity is 25% CC reduction.

Oh, nice okay, Ty (:

I'll do the incredibly hard work of switching to the Announcment section opening the patchnote and find the place where it is explained for you. And i'll paste it here:

New Stat: Tenacity
Tenacity reduces the duration of stuns, slows, taunts, fears, snares, sleeps, silences, blinds and immobilizes by 1% per point. Multiple Tenacity effects do not stack, but Tenacity does stack multiplicatively with other disable reductions (for example Irelia's Ionian Fervor).

so it doesn't also stack with other items havin tenacity?
for example like about UNIQUE Passive?;/

it's kinda obvious, i just ask, doesn't Tenacity stack at all, or does it maybe stack if u buy 2 different Tenacity items, cause ive seen some guys who bought all 3 items:P

As is clearly stated in announcement section, which was copy/pasted here, the tenacity items do NOT stack together.

The single highest value gets taken, it doesn't add them together.

the items have poor stats anyways, having it stack wouldnt even be OP

Get mercs + Moonflair Spellblade + Eleisa's Miracle on a caster (or change spellblade to Cloak and Dagger for a physical) and you get 85% CC reduction... And the total price of all 3 is less than 4k which basically equals one big item. Of course it wouldn't be OP.

Edit: calculated the reduction wrong, brain malfunction