The aim of Socket.IO is to make real-time apps possible in every browser and mobile device. It supports several transport protocols, choosing the best one for the specific browser.
Look up the technical definition for the phrase real-time and you'll see the real-time web is not truly real-time. The actual meaning of real-time involves software with strict time boundaries that must respond to events within a specified time constraint. It is typically used in embedded systems to respond to button presses, for applications as diverse as junk food dispensers and medical devices in intensive care units. Eat too much junk food and you could end up in intensive care, and you'll be served by real-time software in both cases. Try and remember the distinction between different meanings for this phrase.
The proponents of the so-called real-time web should be calling it the pseudo-real-time-web, but that's not as catchy a phrase.
What does it mean that Socket...
