A stateless protocol or system is one where no session information is retained by the receiver, usually a server. Each request from a client to the server must contain all the information needed to understand and process the request. This means the server does not need to store session information about each client for the duration of multiple requests.
The most common example of a stateless protocol is HTTP, where each request is processed independently, without any knowledge of the requests that came before it. This can be beneficial for scalability, as the server doesn't need to retain a large amount of data about each client.
However, many web applications need to maintain state in some form, such as user login sessions. For these cases, state information is often stored on the client-side in cookies or tokens, or managed through a database.