Gather around boys and girls and let me tell you about this old school thing called "crowd control" - aka CC.
Once upon a time the xpacs were new and so players would be going into instances with groups that were composed of toons that either never had been in the instance before or had only been in it a few times. The gear on these toons, while impressive compared to the previous expansion, was woefully under rated for the mobs the players would have to fight, meaning that a tank who tried pulling large numbers of mobs at once, a healer that wasn't fully paying attention or DPS who'd go full burn before the tank had aggro oftentimes brought ruin and failure upon the group.
To compensate for these difficult pulls, players developed a technique known as crowd control in which they'd use the various spells and abilities of their classes to immobilize and silence mobs so as to neutralize their abilities to heal each other or hurt the players' toons.
Mages would do stuff like sheep or silence casters, classes with root or freeze-type spells would lock melee oriented mobs into place so that they couldn't hit anyone, classes with spell disrupt capabilities would have these abilities on their hotkey bar so that they could prevent mobs from casting particularly nasty spells or healing each other, etc.
This would result in one or more mobs being neutralized and/or having their capabilities degraded during pulls, meaning that the players wouldn't have to worry about mobs healing each other, raining magical death upon them, or having three or more beating on the tank because, well, until the gearscore of the entire party is so far above the rating of the instance you're in that mobs healing or putting out massive damages doesn't matter, removing those threats is useful.
Unfortunately, most players are new, have short memories and/or were solidly geared last xpac and as a result, never really had to deal with this "CC thing" or, in d00d-speak - "lol CC wut why?"
For the past year we've all been breezing through instances, barely even stopping to let the healers drink for the simple fact that most tanks were so solidly geared that the healers often times switched to DPS spec mid-instance because they were bored and if a group took more then 15m to finish an instance you put them all on ignore because "20m is too long to take to clear something as simple as heroic HoR - you people suck!"
But things have changed now - healers are leaking mana like a 12 inch bucket with an 11 inch hole in the bottom, the mobs are casting offensive spells and healing like they were being controlled by skilled players and most groups you end up with are only evenly geared (at best) for the instance, with many people only just barely squeaking by the GS minimum (and quite a few doing so by cheating too - ie/queuing geared in the highest stuff they could find even if it doesn't match their spec and then swapping out to lower, but appropriate, pieces once they're in).
As such, learning how to use and actively using the CC/disruption tools available to your class is now vital as too many players are still of the "one wipe and I bail" mindset, especially tanks and healers, who don't have to deal with 30m+ queue times like all the DPS folks do.
I'm sure by spring the overall GS of the average toom in a pug will reach the point that you'll be able to start being sloppy about CC and not waiting for the healers to mana up, but until then, get used to either crawling through instances and succeeding or multiple wipes and nerd rage disbandings, because CC is once again a matter of life or death.