Rogue one is a prequel (OH shit!) and takes place just before a New Hope. We remember that Darth Vader was attacking that rebel ship because it had received a transmission from some rebel spies. That transmission was the plans for the Death Star which the Princess put into R2D2.
Rogue one is the story of those spies.
Rogue one is also the origin story of the beginning of the Knights of Ren.
The Knights of Ren appear in the Force Awakens, we see them only briefly in the movie and they have no speaking roles at all and are rather mysterious.
You have The Heavy, The Armorer, The Monk, I can't remember the sniper's name and the last one is called The Rogue.
Kylo Ren is the leader of the Knights of Ren. We only see them in The Force Awakens in a dream sequence from Rey who is force sensitive for unknown reasons. The Knights themselves are not force sensitive as far as we know.
In Rogue one these are the rebel spies who steal the plans for the Death Star and transmit them to the rebels-

These are the "heroes" of the upcoming Rogue one movie. Starting from the right, the guy sitting there with the staff, he ends up becoming "The Monk" of the Knights of Ren, the next guy standing behind the Monk is the Heavy, the middle character is code named "Rogue One" by the rebels and she is the main character of the movie (and theorized as some as the mother of Rey but that's not true) and she becomes "The Rogue". The guy standing just behind her to the left is the sniper guy, don't remember his name from the Knights, and the guy sitting is the "Armorer", in Rogue One he's the mechanic jack of all trades kind of character.
The story of Rogue One tells the story of the spies who stole the Death Star plans in the very first Star Wars movie and what happened to them. It gets connected to the new movies because in the next installment, Episode VIII it will be revealed that the Knights of Ren are these rebel spies. They were captured after they sent the plans, tortured and turned.
The information we know about the Knights of Ren comes from-

which is canon and features all the Knights of Ren and their names. Combing this with what we know about the upcoming Rogue One it becomes obvious that the Rogue One is really about the birth of the order of the Knights of Ren.