fabric medium is a liquid that you can buy either online or in art stores that you mix with acrylic paint (if you're looking for one on the internet, just search fabric medium for acrylic paint) to make it less rough/stiff and so that it won't peel off as fast. i can recommend https://www.amazon.com/Liquitex-Professional-Fabric-Effects-Medium/dp/B001US2NRO/ as i have used it for some of my school stuff.

hope this makes it more clear