An "agent" in grammar is a person or thing that's doing the action in a sentence. Imagine you're playing pretend and you're doing something like cooking a pretend meal or building a pretend tower. The person or thing that's doing the pretend action is like the "agent" in that sentence.
For example, in the sentence "The dog chased the cat," the dog is the agent because it's doing the action (chasing). In the sentence "The rain made the flowers grow," the rain is the agent because it's causing the action (the flowers growing).
It's important to know about agents because they help us understand who or what is responsible for an action in a sentence. So next time you're playing pretend, think about who or what is doing the "action" and you'll understand what an agent is in grammar!