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

Consider the example below where a vertical force is applied to the beam. Here the dotted line is 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.