Markets are the 'where' of Demand based performance measurement. Demand is a market sensitive measurement which means it will adjust for any given Show depending on your selection of which market the audience for the Show resides.
Markets within DEMAND360 are labeled as countries but it is important to remember that his is where they audience is and not necessarily where the Show of interest is legally distributed or available. Demand is an audience-centric measurement.
You can use Markets in several different ways in DEMAND360:
You can naturally select the Market(s) you are interested in when conducting analysis via the Market Selector
You can explore Markets via dedicated pages
You can specify a Market on carousels so that the rank for Shows is in the market you are interest in.
For a current list of markets available in DEMAND360 Enterprise see here