Rogue One: A Star Wars Story comes out next week, and unless you've been living under a rock, you probably know that the movie involves the theft of the plans for the Death Star. There's also a chance that you've wondered if this movie will finally explain what a Bothan is, and why many of them died to bring you this information.
But we here at The Verge would like to correct this misconception. Rogue One details the story of Jyn Erso and her band of Bothan-less misfits stealing the plans to the first Death Star, sometime before Episode IV: A New Hope. Said plans go on to make their way to Princess Leia Organa, who smuggles them to the Rebel Alliance, where they are analyzed to reveal the crucial flaw of the thermal exhaust port that is later used to (spoilers) blow up the Death Star. But it is crucial to note that — as far as we are currently aware — there are no Bothans in this film (dead or otherwise).
The aforementioned Bothans don’t come into the story until Episode VI: Return of the Jedi, where the Rebel Alliance apparently employs the spies to secure plans for the second Death Star. They also supply crucial general intelligence: the location of the new space station by the forest moon of Endor, the fact that the Death Star's superlaser is not yet fully operational, and that the Emperor himself is onboard overseeing final construction personally. As Rebel leader Mon Mothma makes sure the entire Rebel fleet knows, many Bothans died to bring back that information. (Much of this is later revealed to be disinformation as part of the Emperor's trap to destroy the Rebels, which means the Bothans probably died for nothing, but I digress.)
As to what a Bothan is, and the details of the operation that caused the death of many of them, we turn to the now-defunct Star Wars Expanded Universe, which explains that Bothans are a bipedal race of furry humanoids who are spies — yes, all of them are spies, in a very David Eddings-esque Planet of Hats trope. The details of the second Death Star heist was largely explained in the EU as well, specifically in the Shadows of the Empire multimedia project and X-Wing: Alliance game. Now that the Expanded Universe is no longer canon, Lucasfilm is free to retell that story (along with what exactly a Bothan actually is) as they see fit in the new continuity. However, it’s important to be clear about the fact that Rogue One is not that retelling.
TL;DR: Rogue One is about the theft of the first Death Star plans. No Bothans died or were otherwise harmed in the making of this film. Probably.