I had never seen these before - giant outdoor screened-in porches. They seem to be all over the part of Orlando we are visiting.
This particular one is almost larger than the house itself. (It covers a swimming pool.)
I haven't quite figured out the purpose - mosquitos don't seem that bad to me. Heather said she talked to a transplanted new native, and she told her that they drop the temperature out there by 10 degrees. That would be a good thing.
Update: a friend here told me they are almost exclusively used to cover pools.