"Augmentation" means adding a second medication to boost or complement an existing treatment rather than replacing it. A common pattern is adding low-dose trazodone at bedtime to a daytime SSRI or SNRI — both to help with sleep disruption the primary antidepressant may cause, and sometimes for an additional antidepressant effect.
This combination is well established in clinical practice, but it does combine two serotonergic medications, so it should only be done under a prescriber's direction, with awareness of serotonin syndrome symptoms as a precaution.