The Neutral Axis is the plane across the length of an object such that when a force is applied to the beam there are no tension or compression forces along this plane.

Consider the example below where a vertical force is applied to the beam. Here the dotted line represents the plane showing the location of the neutral axis. All parts of the beam below the dotted line are in tension and all parts above the dotted line are in compression but at the dotted line there are no compression or tension forces.

The neutral axis will depend on which direction the force is applied and will be perpendicular to the direction of force. The side of the cross section experiencing tension and compression will also change based on the direction of the load acting on the object