Mechanism of action refers to the process through which some type of treatment or intervention has an effect on the condition in question.
The mechanism of action of a medication is the specific biological process through which the medication causes the reduction in symptoms. The mechanism of action of a psychosocial intervention (for example, psychotherapy) is the specific intervention that produces change in the client's symptoms.
For many medications, particularly psychotropic medications, the specific mechanism of action is unknown. Similarly, the mechanism of action for most psychosocial interventions is unknown. (I.e., we do not know what specific therapeutic interventions produce the change when someone's symptoms are improved in psychotherapy.)