In telecommunication a duplex communication system is a point to point system composed of 2 or more connected devices.
And it can be:

• Full-duplex system, simultaneously or bi-directional at time
  

• Half-duplex system, communication is one direction at time
  

HTTP 1.1, HyperText Transfer Protocol is a Half-duplex system.

So writing chat applications using i.e. LAMP Stack, (Linux, Apache, Mysql & PHP), it involve:

• Actively sampling the status of the server (Polling) for changes
• Keep track timestamp and
• It’s slower than it should be.

Socket have traditionally been the solution for real-time chat application are architected, supplying a bi-directional (Full-duplex) communication channel between the client and the server. It actually use the URI format.

ws://userinfo@host:port/path?query#fragment 

This is just a start to wrap your mind around in case you want to understand how socket.io

It also contains very helpful examples