"Light pressure" refers to the pressure on the surface of an object when it absorbs or reflects light. Another term is "radiation pressure." This pressure stems from a change in momentum when a photon hits an object and is reflected. The momentum change acts as a pressure (and you will remember that light also acts dynamically). Photons have no mass (weight), but because they have the characteristics of waves, they also have momentum.

The force can be quite significant. The radiation pressure of sunlight on one square meter of the earth is as strong as that from accelerating a one-gram object at seven millimeters per second. In reality, there is little affect on the earth's surface because of air friction and gravity. But the impact on dust particles in space can be massive. So, the concept has been proposed of fabricating a "photon rocket" that would be propelled using this force.