Within the context of Software License Models, a High Water Mark usually refers to the maximum number of concurrent licenses checked out by a given customer over a certain period of time. For example, a customer may have purchased 20 floating licenses for a given software product. Over the last month, for example, the users making use of that software would be checking out licenses and then checking them back in when finished. The licensing system ensures that at no point does the aggregate number of concurrent users exceed the number of floating seats purchased. During a given period though, the High Water Mark represents the maximum concurrent use over that period of time (which by definition will always be less than the number of purchased seats).
See also Check Out, Check In and Floating Licensing