The glass elements which make up a traditional camera lens can be thought of as cross-sections of a large sphere. That is to say that the curvature of each surface is even. The problem with spherical lens designs is that light passing through the outer edges of curved camera lenses focus at different points from light passing through the centre. This causes focus errors, or spherical aberration and other forms of optical problems. Spherical lenses work fine if the recording surface is also spherical (eg: the human eyeball), but in the case of cameras it’s not - the film surface or image chip surface is always flat.

There’s some confusion of terminology here. The word “lens” refers both to a single chunk of shaped glass (think of the single lens of a magnifying glass) and the tube-shaped device containing such lenses which you fasten to the end of your camera.