Spawn works as inoculant for the substrate. It’s actually a small bit of mycelium (preferably from a pure strain) which you spread over the substrate. Here the mycelium has the chance to develop itself. Spawn must be divided evenly over the whole substrate to obtain a proportional growth.
Rye is the most common spawn used. A pot of rye which is totally colonized with mycelium can be an excellent spawn for another 10 pots with rye or a filter bag with pasteurized straw.
If you use spawn make sure that it is totally free of contamination.