Marquis is a french word. It relates to an hereditary title passed down through generations of the same wealthy or noble family.
Think of it as similar to other titles such as Lord or Duke or Earl. In fact it is more similar to Duke and Earl as generally you will be The Marquis of 'Somewhere' as you would also be the Earl of 'Somewhere' or the Duke of 'Somewhere'.
The most famous/infamous Marquis was the Marquis de Sade ('de' is french for 'of') but he, I'm sure, has his own Q+As