Ahh, Kimberly's got it right Horkin, they (there are two of them there) provide an important function! The furnace is equipped with an auxillary return air inlet at the bottom of the furnace for improved natural gravity air flow especially during power outages. The control on the door is tied into the blower circuit so that the door is open most of the time until plenum temp would turn on the furnace blower. When the blower comes on, the door closes to stop any pressurized duct air from leaking backwards out of it. When its open though, return air is largely supplied by this opening from the basement versus the normal return air duct. There are channels inside to put an air filter there. If the filter is in place critters like a cat cannot access the inside of the furnace jacket and crawl into your ductwork. If you haven't put a new filter in, or prefer to run it without a filter here for maximum unrestricted air flow, you need something to block your cat. Mine has a real thing about trying to get up inside there! The dumbells were a perfect fix and rarely get used for their original purpose. I've got nicer plastic coated lighter ones I use these days. As you noticed they provide a secondary occasional purpose of leaning one against the door to keep it closed! A very important accessory!
I took it that she meant this extra one.. However...I can't say what she was thinking because females don't think the same way as us guys.