Haa haa haa this is one of the best questions I have ever seen. Check out "Redneck" in your dictionary. They are very, very ,very simple-minded people. A standard seat is as much as they will ever afford, and getting a row of unoccupied seats on a plane is like first class for them, probably because they have never ever seen the inside of a first class section. Actually, the Americans make a lot of jokes about Rednecks. My favorite joke was about the Australians, when an American on TV remarked about them saying: "These Australians are just a bunch of English Rednecks." Absolute class!

It means exactly what your sentence says: When you are flying in the coach section of an airplane and one or both of the other two seats are unoccupied giving you more room, to stretch, lean or lie down.

I suppose because rednecks are simple-minded people, that two seats for the price of one seems like "first class' to them. ;)