If you're a true Roseanne fan, you know that this episode is actually a very important story arc that played into the final episode of the series. According to the last episode, this episode, the season two finale, is when everything in the series became "scripted," when Roseanne Conner started writing the story of her family.
The actual episode is simple: it's Roseanne's birthday when she finds her old writings and feels inspired to take up writing again, so Dan and the kids turn the basement into her writing studio. But according to the final episode, the real "end" of the sitcom Roseanne was this episode, and everything that followed was all made up by Roseanne Conner. This is Twilight Zone levels of weird.