RxJS is a library for reactive programming using Observables, making it easier to compose asynchronous and event-based programs. It provides core types like Observable, Observer, and various operators to manipulate data streams. Used extensively in frameworks like Angular, RxJS simplifies handling complex data flows and side effects, offering tools for data transformation, error handling, and backpressure management. It enables developers to write cleaner, more manageable code by abstracting away the complexities of asynchronous operations.