Reactive Extensions (Rx) is a programming library designed for composing asynchronous and event-based programs using observable sequences. It allows developers to work with data streams, enabling them to handle events, asynchronous operations, and data changes in a more manageable way. Rx provides a set of operators that facilitate filtering, transforming, and combining these data streams. By using Rx, developers can create applications that respond to user inputs, system events, or data changes in real-time. This approach simplifies the management of complex event-driven programming, making it easier to write, read, and maintain code that deals with asynchronous data flows.