I/O expanders are integrated circuits or modules used to increase the number of input and output (I/O) pins available to a microcontroller or other digital device. They typically use serial communication interfaces such as I2C or SPI to communicate with the host controller and provide additional GPIO (General Purpose Input/Output) pins for connecting sensors, actuators, and other peripherals. I/O expanders are commonly used in embedded systems, IoT (Internet of Things) devices, and industrial automation applications where the number of available I/O pins on the microcontroller is limited.