The agent of a verb (or verbal) is who or what is doing the action represented by that verb.
Jeb built the house.
The house was built by Jeb.
In both these sentences, “Jeb” is the agent. Jeb is not a subject, however, in the second sentence. The noun ‘house’ is the subject in the second sentence. Knowing the difference between subject and agent is important for knowing a verb’s voice (cf. principle 23).