It's in the title... a rocket nozzle with a square shape rather than a circle like all the others. This would help LOADS with making futuristic and scifi spacecraft replicas where square engines aren't an uncommon design choice.
Obviously it's intended as a visual thing so engine performance will most likely be worsened but most people can solve that with some hidden properties abuse anyways.
Its in-game name could be "Futuristic" and have a joke description (seen in career tech tree) of something along the lines of "A ridicilously impractical square exhaust with no benefits... aside from looking cool!"
(i've had this idea for some time and was inspired to post it by that "slanted nozzles" suggestion that got really popular)
I was thinking the same thing. I also need square nozzles.