From Halopedia, the Halo wiki
The fiber-optic probe is a stealthy sensor tool used by UNSC military personnel. Fiber-optic probes are small, wire-shaped devices equipped with cameras on one end, and an input that is plugged into an outlet on a helmet on the other end end, so that the wearer can see through the camera via their HUD.
Fiber-optic probes allow the wearer to see around corners or through small spaces without having to reveal their positions by looking up and exposing themselves to enemy fire. Some of these probes measure at least three meters long.<