The Immobilizer was an experimental piece of technology invented by the Decepticons as a secret weapon. It fired a blue beam of energy that froze a Transformer's servos in place, rendering them incapable of movement.
Stellar cycles ago the ship carrying the weapon, The Harbinger, was shot down by an Autobot squadron when it entered Earth's atmosphere. Starscream investigated the crash site but did not know about the weapon and so did not recover it.
Later, when Airachnid brought the artifact to Megatron's attention he sent her and Starscream to retrieve it as only he knew the exact location of the crash site. After Airachnid located the weapon she tested it out on the pursuing Autobots, managing to immobilize both Bulkhead and Optimus. The weapon was destroyed when Bumblebee was able to fool Airachnid into thinking that he had been frozen as well which allowed him to get the drop on her and shoot it. The components were recovered by the Autobots and were repaired. The weapon was then used to reverse the condition and return Optimus and Bulkhead to their former mobility.