French fit is a sizing method - though the term is a bit archaic.
All statements below are general and there may be exceptions to each.
French fit is supposed to be a slightly less aggressive position than a 'racing' position.
French fit is supposed to be comfortable for longer rides = randonneur distances.
French fit results in less saddle to handlebar drop.
How you can achieve a French fit depends on your preferences, and the bike's geometry. Raising the stem height seems to be the 'standard' and easiest application. Sizing up the frame to gain a taller head tube is another popular way. With the taller head tube the stem does not need to be raised as much, delivering a more tradition road bike appearance.
Every picture tells a story, don't it
^^^ Traditional fit (forgive mismatched tire)
^^^ French fit, note taller headtube, less seatpost.