A lens made of a uniform glass with spherical surfaces cannot form perfect images. Spherical aberration is a prominent image defect for a point source on the optical axis of such a lens. It arises because all rays through the lens are not focused to a common point.
The dependence of index of refraction on wavelength also causes the focal length, and thereby the image position, depends on the color of the light. All simple lenses suffer from such chromatic aberration. This experiment allows you to see both chromatic aberration, and spherical aberrations for a plano-convex lens, and to measure them approximately.