There are a number of legendary items that can drop, some are class specific, some specialization specific within a class. They can drop from completing almost any kind of content and once you've progressed to a certain point you can equip two of them. They're basically powerful items that offer a nice effect or enhance a talent or ability that will allow you to play your class slightly differently with the idea being once you've gotten a few that you'd swap them around to specialise for a certain boss. The big downside is that some are obviously better than others for your spec so there is an element of luck to it as well.
I know people will immediately go 'oh so everyone gets easy legendaries now' but I can't say it bothers me. Ever since Mists with the legendary cloak questline, continuing in Draenor with a legendary ring questline, legendary items have been less exclusive 'special *********' items and more something to work towards that will provide a nice boost, people playing the most would obviously get them much sooner but eventually most active players would too. With Mythic raiding they cater to those wanting to have the unique titles, mounts and armor appearances (at least whilst its current content) so it isn't like they just give everything to everyone for no effort. I know this will be seen as a bad thing by many, then again as I said before, the game is different now, I play it differently too, yes Vanilla-Wrath were some of my best times in gaming, but I couldn't play those again now with the time required.