Generally, this is the use of economic power to seek a higher price than the market would normally support.  Technical economic literature may use this term more specifically to refer to getting exclusions or subsidies through political action.  Understanding this concept is not complete without also looking at how prices can be driven down, particularly in markets for perishable goods.  Labor is a good example as, ordinarily, a worker cannot choose not to produce for any substantial period of time, so the witholding of consumption (of a particular kind of labor) for a time can alter prices to a lower level.  The common component in all these cases is a monopoly on some components of production.  Typically this is based on the artificial scarcity of financial goods when scarce currency is horded by oligarchs.