Blocking I/O is a method of input/output operations where a program waits, or "blocks," until the operation is complete before continuing. This means that when a program requests data from a device, such as a hard drive or network, it cannot perform any other tasks until the data is received. This can lead to inefficiencies, especially in applications that require high responsiveness. In contrast to non-blocking I/O, where a program can continue executing while waiting for data, blocking I/O can simplify programming. However, it may not be suitable for applications that need to handle multiple tasks simultaneously, such as web servers or real-time systems.