To isolate a body under consideration from the other bodies in contact and draw all the forces acting on the body.
For this, first, the body is drawn and then applied forces, self-weight and the reactions at the points of contact with other bodies are drawn.
Such a diagram of a body in which a body under consideration is freed from all the contact surfaces and all the forces acting on it (including reactions at contact surfaces) are drawn, is called a Free Body Diagram (FBD).
For example, if a ladder is placed against a wall with a person standing on the ladder.
In this case, the center of gravity of the ladder will act at the center of the ladder and two reaction forces act normally at contact surfaces of the ladder with the wall surface.