These disembodied hands are the results of the Hand Witch's curse, where she takes the target's hands from them. The Hand Witch can control the hands as well.
The Hand Witch has many sentient hands which have remained by her side and become inhabitants of her mountain.
In "Little Gift Shop of Horrors," in order to try and convince the unnamed traveler into purchasing something from the Mystery Shack, Stan Pines shows him one of the hands that is for sale. After the traveler refuses it, Stan invents a story named "Hands Off," in another attempt to gain some profit. In the story, Stan himself has his hands taken from by the Hand Witch to add to her collection, as a result of him stealing one of her gold watches. After a series of events leading to Dipper, Mabel, and Stan returning the watch to her, he is given his hands in exchange. Right after Stan’s story is over the disembodied hand he was holding prior to his story runs away.
The disembodied hands are pretty much what their name implies: hands without bodies. Some of them have five fingers while others have four fingers and they are in various shades of green. The hand Stan was showing-off to the unnamed traveler has a wrist bone sticking out of it.