Okay kiddo, so imagine you want to buy a toy from a store but you're too shy to talk to the store owner yourself. So, you ask your big sister to go to the store and buy the toy for you. In this case, your sister is an "agente in rebus," which means she's acting as your "agent" or "representative" in getting the toy.
Basically, "agente in rebus" is a fancy Latin term that means someone who acts on behalf of someone else in a specific matter or situation. For example, if your parents hire a lawyer to represent them in a legal case, that lawyer would be an "agente in rebus" for your parents in that particular situation.
Hope that helps you understand, little buddy!