For an over simplified explanation, if a gas is heated until it nearly begins to glow, one atom can emit a light wave and that light wave can strike another atom stimulating it to emit a double amplitude light wave in the same direction. As the light beam continues moving through the excited gas, it grows stronger with every collision. This is light amplification by stimulated emission “LASE." What happens in a neon light and a gas laser is similar.
A gas laser is different from a neon light in that it has a mirror on both ends of a straight tube. The vast majority of light that can be generated is light that can bounce back and forth between the two mirrors growing stronger with every passage. One of the mirrors is only mostly silvered, perhaps 90 percent silvered, so the light emitted from the partly silvered mirror is light that has been generated in parallel lines perpendicular to the two mirrors within. It is not random light that has been “pulled together” The light emitted by a laser is called “coherent” light.