Assuming P (usually written in upper case) represents a force normal to a given cross section.
If a point load is applied to any point of the section, stress concentration will cause axial stress to vary.
The context of the question considers the uniformity of axial stress at a certain distance away from the point of application (thus stress concentration can be neglected).
If a force P is applied through the centroid, sections will be stressed uniformly. However, if the force P is applied at a distance "e" from the centroid, the equivalent load on the section equals an axial force and a moment Pe. The latter causes bending of the member, causing non-uniform stress.
If we assume A=(uniform) cross sectional area, and I=moment of inertia of the section, then stress varies with the distance y from the centroid equal to stress=sigma=P/A + My/I where P=axial force, M=moment = Pe. Therefore when e>0, the stress varies across the section.
