Human computer interaction design (HCID) is the concept of ensuring that it is obvious to the user what they are expected to do next.
Imagine it is the first time you have looked at your website. Is it obvious how you move through the different pages to make a purchase or to request more information? If it is not, you will be losing customers.
The correct use of HCID not only makes your website more accessible and user-friendly but it can result in some impressive increases in sales and enquiries.