What does Tenacity Do + Which are the best Tenacity items?

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So you've seen the items that have the stat "tenacity on it" and it says that it reduces crowd control effects, but what exactly does it mean, and are they even worth getting?

In a nutshell, Tenacity reduces all Crowd Control effects by 35%, 1% per point of tenacity, but getting more than one item will not improve the CC reduction. Crowd Control is any spell that causes you to lose control of your character or decrease in control including silences, stuns, fears, slows, and ensnares.

From left to right, we have Moonflair Spellblade, Cloak and Dagger, Eleisa's Miracle, and Mercury Treads.

Currently, in order of most commonly used to least commonly used are: Mercury Treads, Eleisa's Miracle, Moonflair Spellblade, and Cloak and Dagger. The last two are almost never seen.

Why?

When you pick one of these items, it takes up an item slot. Since every hero needs boots anyway, Mercury Treads lets you save an item slot for something more powerful such as an Infinity Edge. The early magic resist also helps you against your laning opponents and in teamfights.

Eleisa's Miracle sees some usage since it builds from the Philosopher's Stone, which is a good early game gold generating/lane sustaining item.

As for Cloak and Dagger and The Spellblade, they simply aren't effective since they make you deviate far from your normal build just for to gain the tenacity.

So are they worth it?

In a nutshell, yes, but it depends who you're playing as and who you're playing against. Your tank on the team should almost always get Mercury Treads, and oftentimes your Bruisers (melee champs) generally get them too since they'll be in range to actually get CCed. Generally powerful offensive spelllcasters like Brand and Annie choose to get Sorceror's Boots since once they throw down their spells it's okay if they get CC'ed afterwards, and should be hiding or poking instead of jumping into battle anyway.

AD Carries are a little more flexible. Most need to right click repeatedly in order to get the best effect out of the heroes, but they generally also have a large amount of range like Caitlyn or Ashe, and might get away with getting Berserker Greaves instead. However, if the other team is very CC heavy and/or contains anti-carries such as Xin Zhao or single target surpressers like Warwick or Malzahar who target you every teamfight, Merc Treads could be very beneficial.