Could be any number of things - hair follicles, glands, fatty lumps, hernia's etc. In general, if you can get your fingers behind it ( between skin and body ) then it is in the skin, and not usually much to worry about. If it's a pup, get the vet to have a look at it when the jabs are done. If older than that, then would depend on how long it has been there, if it seems painful, being chewed at or licked, getting larger, affecting the skin ( red ) or hair (standing up ) as to whether I would do anything with it.