The show had a love triangle between main-girl Korra, broody boy Mako, and hot-smart-rich girl Asami. The two girls both have a romantic relationship with Mako at different points of the series, and the they have a bit of strained relationship as a result. Towards the end of the series, Korra and Asami start facilitating a non-toxic friendship with each other. This, obviously, envigorated the shipping fanbase around them.
As the series neared its conclusion, the thought of Mako/Korra being endgame in the final episode was pretty common. He was Korra's first and most focused love interest, after all. Mako/Asami seemed unlikely at that point, and Korra/Asami... a vague fantasy.
But then the final episode drops and Korra/Asami end up being the canon pair.
The meme "we poppin the BIGGEST bottles when makorra happens tomorrow" was a post by a Mako/Korra shipper on the /co board on 4chan the day before the finale. When that... clearly didn't happen, the meme'd Pepe reaction that
@Yumelinh posted exploded across the internet.
After the show, there was a large part of the fanbase that insisted that Korra and Asami were just "gal pals," as the depiction of the coupling wasn't super explicit. The creators then came out to say that they intended the developing relationship between them to be a romantic foundation, but they had to write around censorship guidelines in order to make it happen.
My personal take in all this is that... I'm a pushy gay who's all about wlw ships, so I was pleased
I don't have much of an opinion of how well the foundation was laid because I didn't finish the show, but that's a bit less important to me. Also just... Mako was the worst. Maybe he got better later, but I just found him so unlikeable. Asami was also my favourite character so the outcome was great for me.
This is all shit I learned through Tumblr osmosis so please correct me if anything isn't explained properly.