James Webb Space Telescope experts give real-time updates on deployment of the telescopes secondary mirror. The secondary mirror is one of the most important pieces of equipment on the telescope, and is essential to the success of the mission.
When deployed, this mirror will sit out in front of Webb's hexagonal primary mirrors, which form an iconic honeycomb-like shape. This smaller circular mirror serves an important role in collecting light from Webbs 18 primary mirrors into a focused beam. That beam is then sent down into the tertiary and fine steering mirrors, and finally to Webb's four powerful scientific instruments.
About the mirrors: https://jwst.nasa.gov/content/observatory/ote/mirrors/index.html
Where is Webb?: https://www.jwst.nasa.gov/content/webbLaunch/whereIsWebb.html