Hang a towel in a greenhouse and it turns into a curtain. Greenhouses are made of windows, it's all glass, there are no walls. So when you put a towel in there it turns into a curtain. It has no choice but to cover a window, after all.
You're hanging these towel curtains because you need a quiet place to eat a Big Mac (まく). But you're a little ashamed. Big macs aren't really good for you, but this stupid greenhouse has too many windows and everyone can see you stuff your face. So that's where the towel curtains come in.