HAT stands for “Hardware attached on top”. It is a new hardware specification for add-one modules for the new Raspberry Pi model B+. HATs have several advantages compared to older add-on modules for the Raspberry Pi.
The Raspberry Pi PoE HAT powers a Raspberry Pi 4 Model B and Raspberry Pi 3 Model B+ via an Ethernet cable.
Power-sourcing equipment is required on the Ethernet network.
Pi2Grover - Raspberry Pi to Grove Connector Interface Board from SwitchDoc labs
Notice in the diagram above this hat from SwitchDoc labs has a serial connection and several I2C connections.
It also has digital IO Grove Connectors.
For analog input attach the following Grove Connected A/D converter.
Grove - 4 Channel 16 Bit Analog to Digital Converter from SwitchDoc labs
Several IoT devices require Analog to Digital Converter. Here is a partial list;