HMI stands for Human Machine Interface, and- very true to form in the industrial world- is a very literal term. HMI’s allow you (the human) to input information into the machine you’re working on, and observe the current status and functioning of the system’s component parts. Think of an HMI as a touch screen and you’ll be in good shape.
In fact, if you think of your phone as an HMI, you probably use an HMI all the time. Most “Industrial”HMI’s are much more rugged, expensive, and don’t look as nice when compared to “Commercial” HMI’s such as smart phones, iPads, touch screen laptops and more, but they work in very similar ways.
Industrial HMI’s tend to be more rugged simply because they get put through a lot more use and abuse than a typical tablet (more on this below).