Reactive programming is just programming with asynchronus data stream. Data stream is an object that emit multiple piece of data over the dimension of time. Depending on the different language this data stream can get different names, like in Dart they called as Stream while in Rx libaries like RxJS, RxJava, RxDart they are called as Observable.
v == value
time ----1s----2s----3s----4s----5s----6s
data ----v1-------v2----v3---------v4----
This is exactly similar to Futures, but futures can provide data only a single result while Streams can provide you multiple results.