El sujeto agente (the agent) is the person who performs the action in a sentence using the Passive voice. The agent is preceded by the preposition "por" in Spanish. For example:
El contrato fue redactado por la secretaria.
The contract was drawn up by the secretary.
Here, the agent is "la secretaria" = the person drawing up the contract.
Bear in mind in the passive voice there are two types of subjects: "el sujeto agente" and "el sujeto paciente". See Spanish Passive Subject.