Gravelings are invisible creatures that cause accidental death. They only appear sometimes, out of the corner of their eye, to reapers. They live on graveyards. Every some years, they have a free day.
No overt explanation for the origin of gravelings is given in-show, but when Mason kills a human outside of schedule, a graveling emerges from the human's body. This graveling doesn't seem to follow the behavior of the others though, so this could be an aberration.
George Lass, the show's protagonist, is shown to have been able to see gravelings prior to becoming a Reaper, possibly hinting at predestination of Reapers. This is never explored or commented upon. Another character, Ronnie, a mentally ill 20-something, is also shown to be able to see gravelings.