Mon Mothma is referring to the second Death Star. Neither she nor Akbar make mention of new blueprints although we can safely assume they have them.
Specifically regarding the information the Rebels got from the Bothans, Mon Mothma says 3 things.
- They know where the Death Star is
- They know that it's weapons systems are not operational
- They know the Emperor is on board
She finishes by saying that "Many Bothans died to bring us this information"
Akbar then takes over the briefing to talk about the defense shield on the Forest Moon of Endor, says that the Rebel cruisers will create a perimeter and the fighters will enter the Death Star to go after the reactor.
They seem to be employing the same strategy (more or less) as the did in Episode IV (blow the reactor) they just have a clearer shot at it since the Death Star II isn't complete and there are passages large enough to get a starship through.
That said, to answer the question that was asked, the Bothans seemed to come up between Episode v and Episode VI specifically to find out where the Death Star II was being constructed and its current state of completion. That the emperor was on board was an added bonus.
Edit: Here's part of the scene. Apologies for video quality