These convenient devices are used to give HD44780-compatible interface LCD modules a serial interface via the I2C bus. This allows you to control the LCD with only two data wires from your microcontroller (and power of course).
We may send one of two versions of this device - the difference is the controller IC. If you have the PCF8574T. the default I2C bus address is 0x27. If you have the PCF8574AT the default I2C bus address is 0x3F.
A jumper on the back controls power to the backlight, and a contrast potentiometer is also fitted.