This is a model of a stone altar. It is not part of the Tabernacle. It is an example of one of the other displays at this facility and represents altars described in the Old Testament.
These stone boxes are called Ossuaries and contain the bones of Jews. After a body decomposes the bones are gathered together and put in a stone box. The stone box is then moved to another location. Hence the term "bones gathered to the fathers". Stone was used because Jews felt that stone was clean even in contact with dead bones.