Every activity in an organization is part of a process. Processes are supposed to add value, i.e. accomplish something that wasn't done when the process started. Most processes aren't optimal - there's generally some wasted effort, or lost time, or scrap, or miscommunication, or rework. These problems all have costs - some small, and some great.
Fortunately, human beings seem to have a built-in drive to improve processes. Management practitioners have, over the years, developed tools which can be used to systematically improve processes. We've also found that, by focusing on processes, rather than on the individuals involved in the process, we get much more rapid, substantial improvements.
Process improvement is a keystone concept in modern management. Some of its relationships are sketched out in what follows:
Dr. Deming's famous PDCA model focuses on process improvement.
Systems Thinking emphasizes a systems view, rather than a process view, but there are strong similarities between the two methods.
Dr. Genichi Taguchi focused on the losses to society which stem from poor quality, and suggested ways of reducing those losses through a form of process improvement, using designed experiments.