A datum point is a fixed point in the ground that does not move. It is also used to give measurements for example your hieghts and it is also used for your man hole covers.
From Wikipedia:
A geodetic datum (plural datums, not data) is a reference from which measurements are made. In surveying and geodesy, a datum is a set of reference points on the Earth's surface against which position measurements are made, and (often) an associated model of the shape of the earth (reference ellipsoid) to define a geographic coordinate system. Horizontal datums are used for describing a point on the earth's surface, in latitude and longitude or another coordinate system. Vertical datums measure elevations or depths. In engineering and drafting, a datum is a reference point, surface, or axis on an object against which measurements are made.
From Google:
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