'Sung' is the past participle form of the verb 'sing' and is used to indicate an action that has been completed in the past. In English grammar, it plays a crucial role in forming perfect tenses and can also be used in passive constructions. Understanding its function as a participle helps in recognizing how non-finite clauses operate within sentences, as it allows for the incorporation of additional information about time, completion, or state related to the verb 'sing'.