There’s no information in the OP as to the location of the pictured altar, but I notice that the ceilings are very low. That suggests it could just possibly be a private chapel, perhaps a convent chapel, and If so, I imagine it could be a window. Over the years, I’ve seen a few convent chapels in urban brownstones which were often designed with the altar in front of a window, presumably to maximize the availability of natural light. Usually the window was stained glass, but sometimes not, probably as a cost-saving measure. Could this be a photo of one of the latter?
As for what’s covering the presumed window … yes, I suppose it could be a shower liner cut-down to size. It could also be a curtain (silk, nylon, polyester?). Notice what appears to be bunching of the fabric on the rod at the top.