Monsieur is what you say to or about a French-speaking man, the way English uses Mr or sir. You can put it before a family name (Monsieur Dupont) or use it alone when you speak to a man you do not know well. In French writing it is often shortened to M. It feels polite and a little formal. English keeps the French word when the man is French, or when the speaker wants the French flavour. It is not for a woman (that is Madame or Mademoiselle) and it is not a church title. A close-looking word, Monsignor, belongs to the Roman Catholic Church — different job, different word, even though both once meant something like my lord.