It is a laser guide star
It is a 20 watt laser tuned to the sodium D2 resonance at 589 nm.... we project it up and make a spot in the sodium layer 90 km up glow, we then can use that spot to correct for atmospheric distortions with our adaptive optics.... Adaptive optics is limited to less than 1% of the sky since it requires a relatively bright star near the object you are observing.... the laser guide star will allow AO to work on ~30% of the sky.....
That is why Keck has spent millions to get the laser working....
It really has been described as a religious experience to see it in action....
It looks like a solid orange pillar going to infinity...