A warrant canary is like a secret message that people use to tell others that they might be in trouble with the government. Imagine you have a pet canary that lives in a cage in your house. You love your canary and take good care of it. One day, some people who work for the government come to your house and tell you that you have to give them your canary. They say it's because they want to take care of it and make it safe.
You don't want to give away your canary because you think they might hurt it or take it away from you forever. But you also don't want to make the government angry by refusing. So, you come up with a plan. Before the government people take your canary, you teach it a special song. This song means "Everything is okay." If your canary ever stops singing that song, you know something is wrong. It's like a secret code that only you and your canary know.
Now you can tell all of your friends that they should look at your canary's song every day. If they see that the canary is still singing the special song, they will know that everything is okay and you're not in trouble with the government. But if the canary stops singing the special song, it means something bad might have happened, and your friends should be worried.
In the same way, some companies that collect information on people, like internet providers or email services, might be forced by the government to give away that information. But they don't want to make their customers angry by just giving away all their private data. So, they use a warrant canary. They tell their customers that everything is okay as long as the warrant canary is still there. But if the warrant canary disappears (they stop using it), it means the government might have made them give away some customers' private data.
It's like a secret code that companies and their customers use to protect their privacy. Just like the canary's song, it's a code that only the company and its customers know. So, if the warrant canary disappears, the customers know that they should be worried about their privacy.