What are you looking for an explanation of? Is there something about it that you don't understand?
I don't think it represents anything or anything like that, if that's what you were looking for. I think it's just that Clark is poor, hasn't developed the Superman costume as it is (doesn't the "short sleeve" stuff take place 5 years in the past?), and just had the shirts made up cheaply so he had something. Anything.
I read it as kind of like Bibbo's Superman "costume" in Return of Superman, if you've read that. It's just the best he has to work with.