Explain the centre of curvature and principal focus?

Centre of curvature of the lenses are the centres of spheres whose parts form surfaces of the lenses. A lens with both surfaces spherical, has two centres of curvature C₁ and C₂. Meanwhile, the principal focus of the lens is the point of the principal axis, where the light rays parallel to the principal axis converge, when they become incident on a convex lens.