Here's the breakdown:
* Were is the past tense of the verb "to be" and acts as the helping verb.
* Serving is the present participle of the verb "to serve" and functions as the main verb in this phrase.
Together, they create a verb phrase that describes an action happening in the past.
For example, in the sentence "They were serving delicious food," the verb phrase "were serving" indicates the action of serving food occurred in the past.