I've never watched Roseanne...but I'm curious as to what everyone is referring to!
Sorry this took me forever to answer. :P
If it weren't nine seasons long, I would totally say "GO WATCH IT RIGHT NOW." But if you have time on your hands, I really would recommend watching it. :)
[spoilers ahead for people who don't want to be Roseanne-spoiled] Basically, for the entire series, we've followed the life of Roseanne and her family. This included relationships between the characters of Mark/Becky and David/Darlene. (A detail you should know is that Roseanne is a writer in her free time.) At the end of the eighth season, we see Dan (Roseanne's husband) have a heart attack. Fortunately, he survived. At the beginning of the ninth season, Roseanne wins the lottery.
Now comes the finale. In the last ten or so minutes of the episode, Roseanne starts to explain something. We find out that every single episode was written by Roseanne for her book. So essentially, every episode for the past nine seasons was made up by Roseanne. They were all altered events and relationships and people created from actual events/relationships/people. She explains that she'd changed things that she didn't like. In reality, David was with Becky, Mark was with Darlene, Dan died from his heart attack, and Roseanne never won the lottery.
I could be wrong in saying this, but I don't think most people took the finale very well. A lot of people, including myself, were quite unhappy at what happened. I was especially upset by the David/Darlene & Mark/Becky switch and Dan's death.
I hope that explained it! :)