An input stream is a sequence of data that is read from a source, such as a file or a network connection. It allows programs to receive data in a structured way, enabling them to process information efficiently. Input streams can handle various types of data, including text, binary files, and multimedia content. In programming, input streams are often used to read user input or data from external sources. For example, in the Java programming language, the InputStream class provides methods to read bytes from a stream, making it easier to work with files and other data sources.