Know what is G D & T

Geometric dimensioning and tolerancing (GD&T) is a symbolic language used on engineering drawings and computer generated three-dimensional solid models for explicitly describing nominal geometry and its allowable variation

When GD&T is used, the geometric tolerances apply to the features - not the dimensions. Therefore when BASIC dimensions are used with geometric tolerances, several different dimensioning schemes may be used without changing the meaning of the drawing.
GD&T is such a precise language, it involves a great many symbols and terms. Here are some of the symbols that are involved in geometric dimensioning and tolerancing fundamentals with a short definition of each.
Geometric Characteristic Symbols - There are 14 geometric characteristic symbols used in GD&T. They are used to describe size, location, orientation, and form.