Persistent connections are connections between a Web client and a server that can be reused for more than one exchange of a request and a response.
In HTTP/1.0, the default action for the server was to close the connection when it had received a request from the Web client and sent a response. If the Web client wanted the server to keep the connection open, it had to send a Connection: Keep-Alive header on the request.
For HTTP/1.1, persistent connections are the default. When a connection is made between a Web client and a server, the server should keep the connection open by default. The connection should only be closed if the Web client requests closure by sending a Connection: close header, or if the server's timeout setting is reached, or if the server encounters an error.
Persistent connections improve network performance because a new connection does not have to be established for each request. Establishing a new connection consumes significant additional network resources compared to making a request using an existing connection.