Okay, so let's imagine you and your friend have a toy trade. You'll give them your toy car and they'll give you their toy dinosaur. Now, imagine you suddenly decide you don't want to trade toys with them anymore. You put your toy car away and tell them they can't have it because you don't want to trade.
That's what a trade embargo is, but with countries instead of toys. It's when a country decides they don't want to trade certain things with another country anymore. So, they stop buying or selling things from that country. This can happen for lots of different reasons, like if one country is mad at another country for doing something they don't like, or they want to put pressure on the other country to change their behavior.
The problem is, just like with our toy trade, not trading things can make both countries sad. Maybe you really wanted that dinosaur and now you can't have it. And maybe the other country really needed something that the first country was selling. When countries stop trading with each other, it can hurt their economies and the people who live there.