So, if you shine a powerful laser at a reflective sail, the photons bounce off and push the sail forward. You can liken it to wind pushing a sailboat, except the “wind” is actually photons.

This idea is the basis of projects like “Breakthrough Starshot” or NASA’s IKAROS, both of which are designed to accelerate tiny spacecraft to a fraction of light speed using photon power.

However, designs like these tend to consist of thin plastic films with a metal coating (usually something like aluminum). While effective, they tend to absorb some of the light that hits them, which is converted into heat.

Although Larry Niven (and Jerry Pournelle) made more use of the laser cannon idea, in the sense of banks of powerful lasers, the first use of the phrase in a science fiction story was probably in The Furies, a 1965 story by Roger Zelazny appearing in Amazing Stories:

“I will now call the local ICI office and requisition a laser-cannon. They have been ordered to cooperate with us without question, and the orders are still in effect. My executioner’s rating has never been suspended. It appears that if we ever want to see this job completed we must do it ourselves. It won’t take long to mount the gun on your flyer. — Benedick, stay with him every minute now. He still has to buy the equipment, take it back, and install it..."

Here's another quote from the Niven/Pournelle 1974 classic Mote in God's Eye:

"Captain, look," he said, and threw a plot of the local stellar region on the screen. "The intruder came from here. Whoever launched it fired a laser cannon, or a set of laser cannon - probably a whole mess of them on asteroids, with mirrors to focus them - for about forty-five years, so the intruder would have a beam to travel on. The beam and the intruder both came straight in from the Mote.

The basic idea for the laser cannon/light sail propulsion system belongs to Robert L. Forward, who published a short paper Ground-Based Lasers For Propulsion In Space in 1961. (Read a short autobiography of Robert Forward.) Dr. Forward gives a more detailed version of the idea in his 1985 novel Rocheworld; see the entry for Interstellar Laser Propulsion System.

The lightsail phrase was probably first used in Think Blue, Count Two, by Cordwainer Smith, published by Galaxy Publishing in 1962. But the earliest reference to light sails was probably in The Lady Who Sailed The Soul, by Cordwainer Smith, published by Galaxy in 1960.
(The starlight sail.)

"Now this is the way that the foresail works. That sail will be twenty thousand miles at the wide part. It tapers down and the total length will be just under eighty-thousand miles. It will be retracted or extended by small servo-robots. The servo-robots are radio-controlled."