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.
Note: This feature is available in Web Workers
Note: Firefox does not currently support the use of server-sent events in service workers (it does support them in dedicated and shared workers). See Firefox bug 1681218.