What is a decoupled CMS?
A decoupled CMS is a website that using a traditional content management system (i.e. WordPress, Drupal, Joomla, etc.) for content entry but the public viewable part of the website is a static html website generated. Basically, the backend and the front end of the website are separate so your website is more secure and much faster than a traditional CMS that combines the backend and the front end.

Why do is a Decoupled CMS or Headless CMS the future of web applications?
Speed and security. Notice a pattern?

What is Headless CMS?
A modern content management system that is only a content entry point. A headless CMS is designed not to present content. To get content out of a Headless CMS you would typically use a static site generator and an API.

Currently:
Decoupled CMS: Traditional CMS (WordPress) > Static Site Generator > Static Website
Headless CMS >Static Site Generator > Static Website

Traditional CMS (WordPress) = live.website

Decoupled CMS: cms.domain > dev.domain > live.domain

Want more simplification? – core dna has a great article