These are catalogue entries whose nature has not been settled. Something was recorded at a position, with a brightness and often a size, and the source catalogue stopped short of saying what class of object it is. They are kept because they are real observations, and because the cross-references that point at them would break if they were dropped.
Usually because the evidence available does not decide it. A faint smudge may be a distant galaxy or a nearby cluster of stars; a point source may be a star or something much further away with a spectrum nobody has taken. Historical catalogues also carry entries that were never confirmed by a second observer, and those stay on the books rather than being quietly deleted.
Position, and usually a magnitude and an angular size. That is enough to point a telescope, enough to match the entry against other surveys, and enough for it to be found again — which is what a catalogue is for. Classification is a later step, and one that a future survey may supply.