Every one at one time or another has seen or looked through a telescope. Most people do not however know or have any idea how they work. It s basically lenses and mirrors.
Telescopes work by capturing, and rendering light to a common focus point.
Light from a star or a planet goes through, let us say a reflector, and hits the primary mirror. Then the light bounces up into a cone shape. You then use the focuser with an eyepiece and focus on the object. With a refractor, which most people relate to how a telescope looks like, the rendering of light is a little different. No mirrors are used to make this type of scope to work.