Isn't a formal "scientific" designation, it's a way of refering to a mixed group of fossils stuck in one place.
Sometimes the fossils will be all of one type, sometimes they will be of many varietys.
They can indicate different conditions in the environment where they formed. When the fossils are all of one type it may indicate that the original environment contained large numbers of that particular type of animal as might be the case near a Clam, Brachiopod or Crinoid bed.
When there a many types of animals represented it may show there was a varied community of animals or it may be that the animals remains were carried to one spot from a larger area. This might happen due to river or ocean currents.
Some of the most mixed "graveyards" come from places in the ocean where there was a lot of wave action. This can be determined by the condition of the fossils - they will more broken and concentrated than other locations.