A slave laser is a laser which emits on an optical frequency which is dictated by an external master laser (seed laser) via injection locking. Typically, the seed laser is a low-noise single-frequency laser, and the slave laser is a high-power laser. An electronic servo system is usually used to keep the frequency of a resonator mode of the slave laser close enough to the frequency of the master laser to allow for frequency locking.
With the master laser removed or turned off, the high-power laser would typically not even emit on a single frequency, or at least with a much higher intensity and phase noise. Injection locking makes it easier to achieve good noise properties, comparing with the attempt of directly obtaining low-noise performance of a high-power laser. It is also possible to drive multiple slave lasers with a single master laser.