Question

what is pole and focus

Answer 1

Pole is the mid point of a spherical mirror. Principal focus is a point on the principal axis, where a beam of light, parallel to the principal axis, after reflection, either actually meets (concave) or appears to meet (convex).
Hope it will help you.

Answer 2

The principal focus on a convex lense, is where all the light that passes through the lense is refracted to a single point.

The distance between the lense and the principal focus is what they call the focal length.

the point where the principal axis meets the surface