In GD&T, a datum is the origin from which dimensions of part features are established. Datums are based on actual part features or may be simulated by gages or fixtures. When a part feature is used to establish a datum it should be accessible on the part and large enough to be used as a datum. In some cases when a feature is very large or may be irregular by nature, the designer may specify datum targets that will be used instead of the entire feature to create a datum. If a part is intended to be part of an assembly, the best choice for datum features are usually those that touch other parts.
The left figure below shows a part with datum A located at the bottom surface. The right figure shows the datum plane A and how the manufactured part surface establishes this datum. Since no manufactured surface will be perfect, the high points on the surface create the datum. Any dimensions relative to datum A will be measured according to this theoretical plane.
It is typical in GD&T to establish at least three datums in different drawing views in order to properly orient part features that will be measured in 3D.