Product review (4 stars): Replacing this part is fairly easy. On mine
This is an Aprilaire humidifier panel. If you have an Aprilaire humidifier that it fits you'll want to buy one to replace the old one at least once a year. Other than that, there's not much to these. They're a big mass of glued-up material that holds water.

Replacing this part is fairly easy. On mine, you press a lever on the bottom of the unit near the furnace and lift the cover off. Set that aside and you will be looking at the old panel inside a snap-together frame.  The frame comes apart by just prying it in the right direction. Didn't take much. The previous owner hadn't cleaned mine in years, so I soaked mine in vinegar for a while to clean up the scale. Snap the new panel into the frame, put the bottom into the unit and push the top into place. It's just friction-fit in there.  Put the outside cover back on by getting the top slots into place and snapping the bottom in and you're good to go for another year.