Traditionally, a web page has to send a request to the server to receive new data; that is, the page requests data from the server. With server-sent events, it's possible for a server to send new data to a web page at any time, by pushing messages to the web page. These incoming messages can be treated as Events + data inside the web page.
To learn how to use server-sent events, see our article Using server-sent events .
HTML Living Standard
The definition of 'Server-sent events' in that specification.