Craftsmen is a Military Policy which enhances adjacency bonuses of the Industrial Zone. It is also quite an important and useful one, since the Industrial Zone is one of the most important Districts, and it is usually easy to provide it with adjacency bonuses. When you add the bonus from Craftsmen, you will get a considerable boost to all Production in cities with Industrial Zones! Use it well to advance your construction projects, build wonders, or enlarge your military.
The fact that it is a Military Policy also makes this Policy even better. When playing a peaceful game, civs will often have more Military Policy slots than they know what to do with. Craftsmen allows them to get a substantial bonus without sacrificing an Economic slot.
The craft guilds of the Middle Ages not only protected craftsmen (and their customers from shoddy work), but insured that there would be ever more craftsmen. An aspiring craftsman would pass through various levels of training and responsibility – from apprentice to journeyman – before being tasked with producing a “masterpiece.” If that was acceptable, the candidate would be a master craftsman, capable of producing the finest of works in their craft, be that jewelry or carpentry. As new crafts evolved across medieval Europe, new craftsmen swelled the growing cities: cobblers, barbers, brewers, curriers, farriers, apothecaries … a nigh endless list of the middle class.