there seem to be a lot on mushroom growing questions running about on the forum lately... perhaps there should be a dedicated forum/thread....
but yeah, a spore print is obtained by laying down a mature mushroom cap over a surface (usually tin foil or paper) and letting the spores underneath collect there. The shape formed usually is identical to the outlines of the "gills" of the mushroom cap. A print can be used for storing spores, as well as identifying species.