Doubt: Web Sockets and how to use

Question

I was seeing this question link.

Then I did not read in the answer about nodejs and researched about, but I did not understand much. How to apply this technology (nodejs)?
I am developing a site in php/jquery and in it I have an administrative panel with a menu that contains a badge, then I created an ajax function that runs every 3 seconds and if there is a change in the bank it notifies me in this badge. I read that it overloads the server.

That is the question:

How to make this notification more correctly?
If possible, some explanatory example where changing the database or identifying the user’s action on the site, update this notification of mine (badge) on the administrative panel.