Angular is an open source platform / framework for building rich web applications. These applications are typically called 'single page applications' and they provide dynamic and interactive functionality to the user right on their web browser.
Angular comes in two flavors. Angular 1 (also called AngularJS) and Angular 2+ versions (just called Angular). Angular versions 2+ are entirely different from Angular 1. The courses on this page specify which version of Angular it covers, so make sure you take the course on the version of Angular that you are interested in.