Reactive programming has been around for a long time. It’s a paradigm oriented around data flows. The usual example is a spreadsheet: If cell B refers to cell A, then cell B updates automatically when I change the value in cell A. If ten cells refer to cell A, then all those ten cells change automatically. This is as opposed to C++, where if I compute ten variables using variable X, then if I change variable X, I have to manually write code to make sure that the other ten variables get updated.
What’s changed recently is that there are a few interesting approaches that make this paradigm more widely useful. Functional Reactive Programming (FRP) is a useful approach which is spreading, with popular libraries available for pretty much all languages. For an illustration of FRP and how it can be used in an imperative style I recommend reading Deprecating the Observer Pattern.