How do two mirrors help produce an intense laser beam in a solid-state ceramic laser?

In a solid-state ceramic laser, the two mirrors function as an optical resonator that is essential for building up light energy to produce an intense, coherent beam.123

The process works as follows:

• Containment and Reflection: The mirrors are placed facing each other on opposite sides of the laser medium (such as a ruby or Nd-YAG ceramic rod). Light emitted by the excited atoms within the medium travels along the line between these mirrors and is reflected back and forth.13
• Amplification: As the light bounces between the mirrors, it repeatedly passes through the laser medium. Each pass stimulates more excited atoms to emit additional photons, which increases the intensity of the light with every trip.23
• Output: One of the two mirrors is typically completely silvered, while the other is partially silvered (partially transparent). This allows a fraction of the amplified light to escape the cavity, forming the intense, narrow, and monochromatic laser beam.123

Without this resonant cavity, the medium would act only as an optical amplifier rather than a laser oscillator capable of generating a beam internally.3