HMI (Human-Machine Interface) is generally understood to mean a touchscreen whose various pages are designed to make using a machine as straightforward and efficient as possible.
It allows you to enter setting values ​​(used by the machine's PLC program), display sensor values, statuses, etc. The touchscreen can also replace certain physical buttons, which can simplify wiring.
The HMI therefore communicates with a PLC. It is said to be the "master" in this communication, and is also capable of writing directly to the PLC's registers.
The editing software is generally free and intuitive. It also allows the easy implementation of powerful features into your pages.