The Construction Beam is a type of laser beam used exclusively by Sentinel Constructors (Although you can see a Sentinel using it on The Arbiter). The beam itself is very thin, at least 1/2 millimeter wide, and is a light green color. It is super-heated, similar to the Sentinel Beam, and is used strictly for the purpose of repairing Forerunner facilities. It's power is very weak in comparison to its Sentinel counterparts, but it may be focused to allow repairs rather than damage to sentient beings. The Construction Beam is not supposed to be used as a weapon, as it does minimal damage to nearly all organisms, but the Constructor will not stop if something blocks the beam. It is for this purpose that the Constructors count on the Sentinels for protection from hostile forces. The beam is usually used in conjunction with other beams from other Constructors. The beams can be seen being used on Sacred Icon. The beams are being used to repair the sheared off platform you start on and also to open the pistons. On Legendary if a player gets in front of the beam their shields will fall immediately and most likely die if they stay in the way.
The Constructors use this beam for the sole purpose of making repairs to anything that was created by the Forerunners. This can include installations, space stations, and other structures. It is believed that they are also capable of repairing other Sentinels.