A plenipotentiary is a person who has the power to do important things for their government in a foreign country. It's like when you're playing a game with your friends, and you're the leader, and you can make important decisions that everyone else has to follow.
When a country sends a person to another country to represent them, that person is called a plenipotentiary. The government trusts this person a lot, and they give them the power to negotiate with the other country on their behalf.
So, for example, let's say your country wants to make a trade deal with another country. They might send a plenipotentiary to that country who has the power to negotiate and make decisions about the trade deal. This person might be able to make deals that could benefit your country a lot, but they also have to be careful not to make any bad decisions that could hurt the country.
Overall, a plenipotentiary is a very important person who has a lot of power to represent their government and make important decisions in a foreign country.