It is a classification used by the financial services industry to denote an individual or a family with high net worth. The cutoff for this designation is imprecise; some companies use the term to denot individuals with over $100K in assets, other companies $500K. Although there is no precise definition of how rich somebody must be to fit into this category, high net worth is generally quoted in terms of liquid assets over a certain figure. The exact amount differs by financial institution and region. The categorization is relevant because high net worth individuals generally qualify for separately managed investment accounts instead of regular mutual funds.
HNWIs are in high demand by private wealth managers. The more money a person has, the more work it takes to maintain and preserve those assets. These individuals generally demand (and can justify) personalized services in investment management, estate planning, tax planning, and so on.