When a user submits a request to a server, the server assigns a sessionid to each user, stored in the user's browser's cookie, SessionID is global, meaning that as long as the cookie exists, the server will consider it to be the same user, This enables each user to have their own independent global session domain. When the user requests again, the HTTP header to the SessionID cookies to the server side, the server to find this SessionID, if found. It proves that the user's status exists.
We can use the following experiments to clear the understanding session:
Click the button to return the page, but the output is still the same sessionid
In the browser cookies we find the server in the client for our user save the SessionID, each time we request the server with this SessionID as key in the server side of the session to look for our user's session domain.