What does hmi mean?

HMI is "human machine interface" for short. It's the interaction between humans and machines. The goal of this interaction is effective operation and control of the machine on the user's end. The feed back from machines aids the operator in making operational decisions or improvement.

For example, if you want to do detailed control to a machine. For main objects, you need a hmi and plc. You can do the operation on hmi's touch panel, then hmi send the command to plc, and plc deal with and process the command. Without hmi, you can't reach to the detailed controls, and only simple can be done.