In the books, almost all the supernatural creatures on Midgard are able to enter the glom. The vampires can use the factors of the glom to affect red bloods and other blue bloods minds (they can use Telepathy, sugestion, and many other mind related powers), while the witches can see the glom as dark and sandy place with paths, hills, trees, and even owls (possibly spirit animals). The glom is the place were the souls of the deceased appear before crossing the gate that leads to hell and Helda forever. In this place, the souls still have the opportunity to return to the world of the living if someone decides to resuscitate them.
In the tv show, the glom seems to be the place where Freya and Frederick enter every time they do the astral projection spell. This is plausible given that the way they enter in the glom is the same as in the books.