As an example a chat on a website:
You can write something yourself and a message is displayed directly without having to refresh the page.
Quite simply like a stream chat on youtube or twitch.
How do you program this?
Asynchronous Javascript (Ajax), so that you can eg call PHP functions on the server and display the returned data without the whole page needs NEN refresh
Either you use WebSockets or comet techniques (long polling with AJAX, SSE).