Okay kiddo, imagine you have a toy that your friend always wants to play with. They keep asking you nicely to let them play with your toy, but you really like it too and don't want to share. Eventually you might give in and say okay, but only for a little while.
In ancient Greece, there were two big cities called Athens and Sparta. Athens wanted to control a small island called Melos, but the people of Melos didn't want to be controlled by Athens. So Athens sent a group of people to talk with the people of Melos and convince them that it's better to be friends with Athens and be protected by them. This conversation is called the Melian Dialogue.
Now, the people of Melos knew they were in trouble and didn't have a lot of power to resist Athens, so they tried to argue with the Athenians and convince them that it's not fair for them to just take over the island. The Athenians listened, but then basically said "tough luck, we're more powerful than you and we're going to do what we want."
The Melian Dialogue shows how sometimes in life, the people who are stronger and have more power can do what they want without really listening to others. It also shows that sometimes, no matter how hard you try to argue or find a compromise, the other person or group may not be willing to budge.