Every structural member has its strong as well as weak axis.

A beam will fail easily along the direction of lower moment of inertia, a column will fail about the axis with lower radius of gyration.

So the big question is how to find a strong or weak axis for the given cross section.

Now if you are given a known cross section e.g.wide flange beam, rectangular column, then you can calculate the moment of inertia of the cross section easily & then can define which one strong axis and which one is weak axis of the given cross section.

The direction in which you have larger moment of inertia is a strong axis while the perpendicular direction to that is a weak axis.