Okay kiddo, have you ever picked grains like wheat or rice from a farm? You know how they have those hard outer shells that we don't eat? Well, farmers need to get rid of those outer shells before they can use the grains for food.
A threshing stone is a big rock or a heavy piece of metal that farmers use to remove those outer shells from the grains. They put the grains on the ground and then use the threshing stone to hit them over and over again until the shells break off.
It's kind of like playing with play-doh and using a rolling pin to flatten it, except instead of play-doh, it's grains and instead of a rolling pin, it's a big heavy rock or metal.
Threshing stones aren't used as much these days because of technology, but they used to be really important for farmers to get their grains ready for food.