Degrees of Freedom! 3-2-1 principle in GD&T! Welding Fixture Design Basic! By Aalok Sharma 

The degree of freedom defines as the capability of a body to move. Consider a rectangular box, in space the box is capable of moving in twelve different directions (six rotational and six axial). Each direction of movement is counted as one degree of freedom. i.e. a body in space has twelve degree of freedom.
In 321 principle, the primary (usually a plane) locks 3 degree of freedom, 2 rotations and 1 translation respectively. The secondary locks another 2 degrees of freedom, 1 translation and one rotation. Finally the tertiary datum locks the final translation. In 321 all the datums are mutually perpendicular to each other.