It’s dark grey polyurethane foam attached to a rubbery magnetic backing. You’ll need to trim it to your air conditioner. Use a sharp blade because it’s not very easy to cut through cleanly. Also, if your air conditioner has a top vent, be sure to cut out a hole for that. But it works! No more maddening ping pangs from rain or upstairs neighbors’ AC drips. The magnet works very well to keep it in place, so I’m glad no glue was necessary. I’m taking one star off because it’s a bit expensive for what it is, and I’ve read in another review that it eventually loses efficacy and needs to be replaced. I can certainly imagine it getting filthy by holding in water like a sponge. ..More