I don't understand lenses

A convex lens is fatter in the middle than it is on its edges. A concave lens is thinner in the middle. A convex lens will focus parallel rays down to a spot at a distance which is the focal length. You can measure the focal length of a convex lens by using it to set fire to a piece of paper on a sunny day. Rays from the sun are pretty well parallel. The distance between lens and paper when the smoke starts is the focal length. There is much more to this subject of course.

oh I see..

thank you very much that was very helpful :D