This is the ship of the (spoilers) Armored Titan-shifter, Reiner Braun, and Historia Reiss/Krista Lenz. On how this ship was born, I believe it was when Reiner said in the anime, "I wanna marry her", and basically the fangirls exploded by shipping it.
The Ship Name
The ship name is kinda complicated. The R/I came from Reiner and the K/RI from Krista, but where did the U's come from? I honestly don't know, sorry.
The Top and the Bottom
This ship like all straight ones; Reiner is seme while Krista is uke.