Ground-based optical telescopes suffer degraded image quality due to turbulence in Earth's atmosphere. Without any correction, the angular resolution of a 20 meter telescope is no better than that of a 20 cm telescope (the larger telescope can resolve much dimmer objects, but not smaller objects).

Essentially, turbulence distorts the (ideally flat) wavefronts of incoming light. Instruments designed to reduce this distortion are called adaptive optics. The AO system usually physically distorts some optic in the telescope to reshape the incoing wavefronts. Learn more about AO on Wikipedia.

Astronomers need a bright point source to apply adaptive optics techniques. The image produced by the source is used in a feedback loop: the deformable optic is distorted to minimize the apparent size of the bright image. To image a dim object, a bright, nearby guide star is used for AO feedback, producing a corrected image with both sources. Unfortunately, these bright sources don't occur near most of the interesting objects in the sky. To perform AO observations in the rest of the sky, astronomers rely on what's known as a laser guide star: a point source produced by laser light backscattered off of the sodium layer in the upper atmosphere.

While this method is simple in principle, there are many experimental challenges to overcome.