If I had to describe HCI is one word, it would be usability. Usability refers to the ease of use for the software/hardware.
So what is usability? Usability asks if the product, webpage, etc. is effective, efficient, and user friendly. Were the goals met? Are the users satisfied? Was cognitive load reduced where possible? Error free experience? Comfortable environment?
Thus usability is extremely important when designing anything. So how do you test usability? Well there are many types of tests that can be performed from experts evals, groups, users, etc. There are also many types of guideline charts out there in the research depending on what type of material you are testing (i.e., a webpage, mobile app, control scheme, etc.).