How its classifieds?
A Datum is a plane, axis, or point location that GD&T dimensional tolerances are referenced to. Typically, multiple features will be referenced by each datum, so they're a very important part of the whole thing. Datums tell us where to measure from.
In the drwaing below, A and B are the Datums used to establish the center of the hole using the Position Symbol and the Feature Control Block for the hole center..