In Java, file input/output (I/O) is a mechanism used to read data from a file or write data to a file. This allows developers to create applications that can store and read information from files, which is useful when working with large amounts of data or when creating applications where data needs to be shared between different users.

The Java file I/O API provides two main classes for working with files: the FileReader and FileWriter classes. The FileReader class is used to read data from a file, and the FileWriter class is used to write data to a file.

In order to read data from a file, you first need to create an instance of the FileReader class and pass the name of the file as a parameter. You can then read data from the file using methods such as read(), read(char[]), and readLine().

To write data to a file, you first need to create an instance of the FileWriter class and pass the name of the file as a parameter. You can then write data to the file using methods such as write(), write(char[]), and write(String).

Additionally, Java provides several other classes for working with file I/O, such as BufferedInputStream, BufferedOutputStream, FileInputStream, and FileOutputStream, which provide additional functionality such as buffering and binary read/write.

Overall, understanding file I/O in Java is an important skill for any Java developer, as it allows you to create applications that can store and share data more effectively.