Webb's mirrors capture and direct light from the distant universe to create the telescope's images. Webb's primary mirror — with its vast expanse made up of 18 segments — gets most of the attention, but several more mirrors help control the flow of light through the telescope. Webb has a secondary, tertiary, and fine-steering mirror, each appropriately shaped for the action it must undertake. The mirrors' ultimate goal is to get the light to the telescope's detectors in order to record and translate it into imagery.
Wikipedia's entry about Reflecting Telescopes
NASA and Mary Estacion (STScI)
Publication: May 14, 2010