Java I/O, short for Input/Output, is a set of classes and interfaces in the Java programming language that allows developers to read from and write to data sources like files, network connections, and memory. It provides a way to handle data in a structured manner, making it easier to manage information flow in applications. The Java I/O framework includes various components, such as InputStream and OutputStream for byte data, and Reader and Writer for character data. These classes enable efficient data manipulation, supporting operations like file reading, writing, and buffering, which are essential for building robust Java applications.