According to Wikipedia: A shadow person is the perception of a patch of shadow as a living, humanoid figure, and, interpreted as the presence of a spirit or other entity by believers in the paranormal or supernatural.
I admit that, since I began Sun-gazing about 6 years ago, I have experienced a greatly increased number of these sightings. I believe the reason is that Sun-gazing enhances periphery vision, enabling you to see things you would otherwise miss. In my case, the figures are not always humanoid, but often they are winged creatures darting by in an instant.
This week I re-watched an episode of the 1985 Twilight Zone series titled "The Shadow Man," which dealt with a teenage boy who had a Shadow Person living in his bedroom. The episode portrayed the Shadow Man as fitting the "Hat Man" appearance often ascribed to shadow people.
According to Natalia Kuna, a psychic medium, "Shadow People are said to be conscious, intelligent, inter-dimensional beings that can shapeshift into various forms and figurations, and move back and forth between dimensions." In 2001, Heidi Hollis published her first book on the topic of Shadow People. She describes Shadow People as dark silhouettes with human shapes and profiles that flicker in and out of peripheral vision. She believes the figures to be negative, alien beings and suggests repelling them by invoking the Name of Jesus.
Personally, I have never had any reason to fear them, although I sometime wonder if there is a message I am supposed to be picking up, but miss. If you are seeing Shadow people and they bother you, start by having a talk to a reputable psychic to determine what the cause might be.