Neutral Axis:

If you refer through Figure (d), the side AD shall be in tension whereas the side BC shall be in compression. The fibres along the side AD elongates, whereas the fibres along the side BC shorten. Also there presents one fibre in between these two faces that neither have extension nor compression. This layer is called as the neutral layer. The line of intersection of neutral layer along the plane of cross-section, of the member is known as the neutral axis. For members subjected to bending just, the neutral axis passes through the centroid of the section. At the neutral axis, the stress shall be zero.