Mediums are able to see and comprehend non-corporeal spirits. A medium's main skill is in contacting the spiritual world, a dimension beyond the human world where all spirits (animal and human) go when the life force is completely extinguished and they leave the corporeal plane. They can actually communicate with these spirits due to their strong psychic connection with spiritual influence.
According to Billie Dean, mediums are being surrounded with white spirit light that protect them from malevolent spirits.