Mr. Poopy Pants Elon has a computer called a server on the Internet that lets you and your friends send notes to each other, but that meanie Mr. Elon reads every note and decides if your friends can see it an if you can see notes your friends send to you. He also has a website where you read the notes and he can block you from using it forever if he wants to.
With nostr there are hundreds of different servers (called relays) you can send your notes to. You and your friends can even make your own. And there are lots of different websites you can use to read the notes, all made by different people. You can can make one those too if you want to. So if someone starts acting dumb like Mr. Elon you can just switch to a different relay or website and all your notes will still be there, and nobody can stop you from sending or reading what you want.