In Reinforcement learning, the agent is one who takes decisions based on the rewards and punishments. Consider an example of a batsman in cricket. He tries to hit the ball if he misses he gets a negative point. If he hits the ball then he gets a reward. So, from these positive and negative experiences, he will understand how to play that particular ball. In this example, the batsman is an agent.
If you are building a drone that can deliver the products from point A to point B. In this case, the ml models are agents.
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You can know more about Reinforcement learning from this video: