The speaker keeps asking us to look at a piece of glass, then admits there is nothing to see. The glass is clear, so it shows nothing, and the poster that once seemed to sit behind it is gone. Or perhaps the glass itself has been emptied. The poem never settles which, and that uncertainty is part of its unease.
Halfway through, the focus shifts from the glass to time. Time is described as short, near, little, impatient, deaf and dumb, a string of phrases that piles up like a nervous list. Then time becomes "a cloak," something that covers or hides. Judging by this, the speaker may be thinking about loss, about something that was once there and can't be brought back.
The mood is bare and hollow. Short lines, repetition and the flat "No. Nothing." give it a quiet, stunned feeling. By the end, the speaker is still circling the same empty glass.