Reflecting telescopes, also known as reflectors, use a set of mirrors instead of lenses to gather and focus light. The primary mirror (usually a concave mirror) gathers the incoming light and reflects it to a focus point. The secondary mirror is used to redirect this focused light out to where it can be conveniently observed.

In other words, the secondary mirror in a reflecting telescope is used to move the eyepiece outside the telescopic tube, where the image can be comfortably viewed. This is especially important for large telescopes, where the focus point may be inside the telescope tube and inaccessible without a secondary mirror.