This Military Policy replaces the earlier Maneuver, and has the same effect, but for more modern units: Knights and Cavalry (and Coursers and Cuirassiers in Gathering Storm), as well as their unique replacements. Note that if your technological development hasn't kept up with your civic development, and you haven't yet unlocked these units, researching Divine Right (the Civic which enables Chivalry) will also make Maneuver obsolete, and may hinder you more than help you! However, Divine Right is a very important Civic for other reasons, so you may not have much choice. In this case, try to make good use of Maneuver before you finish researching Divine Right, and then try to unlock more modern cavalry ASAP.
Before it was reshaped by Christianity, our romantic notions of chivalry were introduced to Europe by the Moors of Iberia through their tales of knightly adventure for the sake of glory and love; once 'The One Thousand and One Nights' was translated into French in the 12th Century, there was no putting the genie back in the bottle. More often idealized than practiced, Charlemagne’s chevalerie (“horse soldiery”) believed chivalry to include a code of conduct that elevated loyalty, honor, forbearance, hardihood and largess above all other knightly qualities (like trampling the godless). All those things that nutcase Don Quixote was raving about.