It means "The Mantle". The color dictates the action. If you press the finger pad of a Forerunner control panel, and make a circle with your fingers (Without removing your finger from the controls, i believe, so continuously) it will come up with similar shapes and glyphs like Google does with Google Instant. If you select the mantle, it will bring up the database first, and this symbol changes color in segments, if you select the light blue, it open transmissions and from there it is voice activated, also as a bonus fact, Forerunners designed all their Holopanels to be Human/Forerunner friendly only, meaning that no other race can access databases or activate any kind of mechanism without the aid of a Human or Forerunner.
If you want to call bullshit, my source is The Halo Encyclopedia, written mostly by Frank O'Connor, Franchise director for Halo.