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The light from a laser is released as a thin beam concentrated on one spot. We say that the beam is highly collimated. This occurs because the cavity of the laser has two nearly parallel mirrors on the front and the back from which the atoms bounce back and forth. The back mirror is nearly perfectly reflected, while the front mirror is about 99% reflecting, letting out 1% of the beam. This 1% is the beam that we see. While the light is bouncing back and forth between the mirrors, it is gaining being amplified due to stimulated emission. This is displayed in the figure below.

Figure 3

Summary (New explanation)

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