As it names suggests, HCI is a field that attempts to understand the way we interact with machines. The idea is that by understanding this relationship we should be able to build better designed software/hardware (or more usable, as it's known in the field). It's a merging field involving mainly psychology, design and computer science. As for how it applies to the real world, either UX design or usability engineering are probably the most commonly found examples out there.

I think it is about making a good design of your product.