I'm a little surprised by this statement. Obviously all my text books are wrong.
I was under the impression that grain is all too real. Basically, a film is a coating of crystals in a carrier, classically gelatine, onto a substrate. When enough photons hit enough molecules within the crystals, the crystals become developable, a suitable reagent turning the molecules of the crystal from the halide into pure silver.
Two preconditions cause the appearance of grain. The first is that the individual crystals are typically irregular and the second is that the coating is generally irregular, so that developed crystals appear as clumps with gaps between them. As grain is not always what the photographer want, manufacturers have various strategies for reducing this clumping, including spreading fewer crystals through the coating; using more regular shaped crystals, as in Kodak's T-Max emulsions, or using a process where all the crystals are replaced by dyes, as in all colour emulsions but also in films like Ilford's XP-2.