`InputStreamReader` is a class in Java that bridges byte streams and character streams. It allows you to read bytes from an input stream and decode them into characters using a specified character encoding, such as UTF-8. This is particularly useful when you need to read data from sources like files or network connections that provide data in byte format. By using `InputStreamReader`, developers can easily handle text data in their applications. It is often used in conjunction with other classes, such as `FileInputStream` or `BufferedReader`, to enhance performance and simplify the reading process. This makes it a vital tool for managing input in Java programs.