Think of it this way - when you meet someone new, you both want to figure out the best way to communicate with each other. If you both speak the same language, great! But if not, you'll need to negotiate and find a language that you both understand.
In the same way, when your computer wants to talk to another computer over the internet (like when you're browsing the web or sending an email), it needs to figure out the best way to communicate. This is where application-layer protocol negotiation comes in.
An application-layer protocol is like a language that computers use to communicate with each other. There are lots of different protocols, like HTTP (which is used for websites), SMTP (which is used for email), and FTP (which is used for file transfers).
When your computer makes a request to another computer (like asking for a webpage), it first sends a message saying "Hey, can we talk using protocol X?" The other computer responds with either "Yes, I speak protocol X too!" or "Sorry, I only speak protocol Y. Can you talk to me using that instead?"
This negotiation process continues until both computers agree on a protocol they both speak. Once they've agreed on a protocol, they can start communicating using that protocol, like sending webpages, emails, or files back and forth.
So in summary, application-layer protocol negotiation is like figuring out the best way for two computers to talk to each other, like negotiating a language when you meet someone new.