Interacting with a mobile app. One of the best bits of digital media is, instead of passively watching, reading or listening, the page can respond to your actions. Moving, scrolling and zooming elements around the page entering and editing data, picking preferences and more can all be done with interactive digital media.
Simple interactivity can be done with the Cascading Style Sheet (CSS) language used on web sites. A simple example is the mouse over highlights of the links in the nav bar above, but for more complex interactions you will need to learn a programming language.
It is most commonly used to make sure you get the data on your web pages you are interested in, customising and personalising your experience. For example, if you are reading a newsfeed, PHP can be used to only show news you are interested in.