Seed Chocolate is chocolate that has already been tempered and successfully formed the desired crystals in it. It is used to add to another batch of chocolate during the tempering process to encourage crystals to form in it.
You can use any chocolate (provided it is chocolate that was previously tempered.) You just grate it and it becomes your seed chocolate.
Once the chocolate that you are tempering is melted and you are starting the cooling process, you sprinkle your seed chocolate in. You don’t need much; only about 1% of the chocolate quantity being worked with overall.
Adding Seed Chocolate helps tempering happen faster and more reliably.