A Raspberry Pi HAT (Hardware attached on top)is a type of add-on module that extends the functionality of a Pi for specific tasks. HATs can be used for IoT applications, PoE, clocks, sensing, displays or cooling – there is a huge range to choose from.An advantage of using a HAT over other add-on modules is that you simply have to plug and play. A pHAT is the same however it is designed for the Raspberry Pi Zero.The Raspberry Pi comes equipped with GPIO (General Purpose Input Output) which allows it to interact with other electronic devices and hardware through the use of programmed code.