Scripts talk to servers in the background so the page never reloads, a technique still called AJAX (Asynchronous JavaScript And XML) although payloads are now mostly JSON. Clients, Servers and HTTP covers the HTTP underneath.
Most requests are pull: the browser asks, the server answers. Push lets the server send news at once. Before browsers had push APIs, Comet techniques faked it with polling and long polling, where each request stays open until data arrives and the browser immediately asks again; long polling remains Socket.IO 24,482 's fallback transport.