Ionic is a framework of PhoneGap which is based on AngularJS and very popular or we can say most popular framework of mobile development using PhoneGap. So basically we can say that Ionic is a angular based framework for mobile development. Using Ionic you can manage API access very well , manage app designs, create app very fast and many more. If you've good knowledge about ionic then you can make a simple app with in few hours(I'm only talking about simple app).
As I told in first that Ionic is based on the AngularJs, you must have to good knowledge of AngularJS. CSS and HTML is the heart of PhoneGap development, so you must have to great knowledge about these. Ionic also provides you CSS file (which is build on SCSS), If you have knowledge about SCSS the it's great and if not then don't worry, you can manage all things by CSS. Ionic also provides some HTML custom tags like : ion-slide-box, ion-slide, ion-content, ion-view, ion-header-bar, ion-footer bar and many more, you can directly use them, these tags are for special purpose, so you must have to read about that.
As I told you at first that it's most popular framework of PhoneGap so if you are working on PhoneGap or planning to work on PhoneGap then you must have to go with this.On internet this is a lot of content/support available for ionic development and most plugins of PhoneGap are available for ionic(the plugin js file customized for ionic), so you can directly used them or if any plugin not available for ionic, you can directly use PhoneGap plugins but you have to apply some extra efforts for this.