Spawn is a vehicle of transfer for the mushroom mycelium as it is
dispersed and mixed into a fresh substrate by the cultivator. Depending
on the substrate to be inoculated, the vehicle can be grain, sawdust,
wood chips, dowels, or rope.
Mycelium is the vegetative part of the fungal organism;
mushrooms are the fruit, or the reproductive (spore-producing) part.
Just as you must grow a tree to produce an apple, you must grow
mycelium to produce a mushroom. In nature, mycelium is seldom seen, as
it will dehydrate if exposed to sun or wind. So it spends its life
within the earth, inside a log, or under some other kind of protective
mulch.
Using spawn to grow mushrooms is a method of propagation closer to
growing apples, potatoes, asparagus, garlic, or strawberries than to
sowing seeds, in that it involves expanding the tissue of a particular
individual, so that you are producing genetic clones of the original
specimen.