You can read the full notes here, but here's the important stuff:
1. New rotating game mode on weekends!
This is a new feature will offer at least one game mode per weekend in addition to the classic and ARAM modes, starting on Friday afternoon and ending early Monday morning. It'll bring some old modes like Legend of the Poro King and One For All back as well as introduce some new ones.
Champion masteries and Hextech drops will be enabled for rotating game mode, which will begin with Ascension after Patch 6.7 drops.
2. Mid-game death timers shortened
If you play League of Legends regularly, you've almost certainly experienced this in recent weeks: You're winning a game, even dominating, for the first 30 or so minutes. Then your team gets caught out of position, you lose a dumb team fight, and all of the sudden the game is over before you've respawned.
In Patch 6.7, death timers from 30-55 minutes have been reduced. Timers before and after those points are unchanged, but that'll help reduce some of the more dramatic, nothing-you-can-do-about-it swings seen in recent patches. Just ask TSM.
3. Poppy got nerfed super hard
This makes a lot of sense from a balance perspective: Poppy was unquestionably the most powerful top lane pick in both the professional and amateur scenes due to her strength in basically every aspect of top lane play. But it's still sad to see as someone who has played a ton of Poppy recently.
The biggest reductions came to her range, both on her Buckler and her Heroic Charge ("E" ability). Poppy's passive also no longer resets her basic attack, which will make farming a little less easy (and her all-ins a little less potent).
4. Maw isn't quite so super strong anymore
Maw of Malmortius has been one of the strongest items recently, offering protection from magic damage while also boosting attack damage. It's been particularly useful on high attack damage champions like Graves, Kindred and Talon, allowing them more survivability in fights, but it's also been utilized on tanks to give some extra punch along with the helpful defensive stats.
Maw's magic resist has been reduced, and the attack speed bonus has been removed. It will likely remain a pretty common item, especially against heavy AP compositions, but it's definitely veering back towards being situational rather than a must-buy for all attack damage champions.
5. Warlord's Bloodlust is less good on ranged champions
Originally intended for fighters like Olaf and Aatrox, the Warlord's Bloodlust change we saw in Patch 6.4 instead saw a lot of marksmen picking it up. Now the keystone mastery's healing from minions has been halved on ranged champions, so we might see less of it on AD carries.
League of Legends Patch 6.7 should be live either later today or later this week.