Mushroom spawn contains the vegetative growth of the fungus on specially prepared mediums. They are created in laboratory conditions so that there are no contaminations that can compete with the mushroom variety that we want.
Spawn comes in different form and are used to introduce the mushroom specie to a bulk substrate such as straw, logs, woodchips and garden beds.

Grain spawn is a blend of grain and other ingredients used as fodder for the mushroom mycelium. The grain is very nutritious for the fungus mycelium and is used to propagate the fungus onto a large variety of bulk substrates or to make more grain spawn and sawdust spawn.
The usual bulk substrate used for oyster mushroms is straw, that has been cold or hot pasteurised.
It is usually used on a bulk substrate at a ratio of 1:20 to 1:40 depending on skill.