This is used as a striker for the tuning fork to activate the sound. It's a hockey puck, so if you happen to have one hidden away in a closet somewhere there's no need to purchase this one. However, if you don't, you might as well grab this one. You'll need it!

Product Link

https://www.omnivos.com/tuning-fork-activator-open-bag.html?ref=frequencyfamily