what is the most significant thing or event in the chapter 34 of emma?

Austen’s use of three chapters to narrate a single dinner party marks an interesting narrative development for English literature. In novels by previous writers, the description of the events of a dinner party would have taken up at most a page or two, but Austen turns the dinner party into an opportunity to trace extensively the ins and outs of human personality and interaction. In doing so, she provides a model for later writers as disparate as Henry James and Virginia Woolf.

I agree with Aslan that the dinner party in itself is the most important event, but I'd like to add that this is where we really get to meet and understand Jane. As a character, we haven't known much about her up until this point, but through the dinner conversation we learn a lot about who she is.