Server-Sent Events (SSE) allow a web page to get updates from a server.

Whenever we perform some event and send it to the server such as by submitting the form to the server. So such type of event which flows from web browser to web-server are called as a client-side events. But if the server sent some updates or information to the browser, then such events are called server-sent events. Hence A server sent event occurs when the browser automatically updated from the Server.

The Server-sent events are mono-directional (always come from server to client). Or it may be called as one-way messaging.