A laser is a machine that makes an amplified, single-colour source of light. It uses special gases or crystals to make the light with only a single color. The gases are energized to make them emit light. Then mirrors are used to amplify (make stronger) the light. In many lasers all the light travels in one direction, so it stays as a narrow beam of Collimated light that does not get wider or weaker as most sources of light do.