Okay kiddo, have you ever made a promise to one of your friends to do something or not to do something? That's kind of how den/ice agreements work between countries.
So, let's say that two countries, let's call them A and B, have some issues between them. Maybe country A has been polluting the water that country B depends on, which makes country B very angry. They might decide to sit down and talk about how they can solve this problem without fighting.
During these talks, they might make some promises or agreements to each other. For example, country A might promise to stop polluting the water or to clean it up, and country B might promise not to take any aggressive actions against country A.
These promises are called den/ice agreements, and they work like a contract. Just like when you make a promise to your friend, if you break your promise, they might not want to be your friend anymore. Similarly, if one of the countries breaks their promise, the other country might get very upset and this could lead to more problems.
So, in summary, den/ice agreements are like promises that countries make to each other to solve problems peacefully. They're important because they help to prevent conflicts between countries and keep everyone happy.