It's not a stupid question Paula....ask away
The first time I saw that in somebody's tank I asked the same thing.
It is a small spawning mop. I use it to catch and hatch out Rainbow fish eggs.
The fish like to spawn over plants .. the yarn mimics plants ..
Rainbow fish eggs have will adhere to plants . Hopefully the Bows will spawn over the mop .
I take the mop out give it a quick dip in a water and methylene blue solution to prevent fungus, and place it in a separate hatching out container ..
I usually use a small tank with a small sponge filter and a wad of Java moss for hatching out any fry.