Mostly clothes. Take a look around at images of medieval costume (like the harvest in Queen Mary's Psalter), and you'll notice a suspicious lack of short sleeves. A full shirt and a brimmed hat or a hood will protect you from the sun quite well. This is actually the general dress for outdoor workers (especially but not exclusively those with fair skin) regardless of time period. Compare the traditional image of the cowboy, for example: long pants, long button shirt, brimmed hat, even in the Texas Panhandle in the summer. (And believe me, it's hot there.) These costumes are all specifically adapted for sun protection.

A modern person will typically object "but that's so hot". Really, it's not so bad with appropriately constructed clothes. Modern cotton clothes perform very poorly with sweat, but a light linen or wool shirt, more common historically, really does quite well. I've worn such an outfit in Death Valley in the summer, and I actually felt cooler in long sleeves than when changed into a t-shirt. It may help to think of a shirt as portable shade.

There are of course, physiological adaptations as well; when one spends lots of time in the sun the skin adapts and develops more melanin for protection. (Tanning, in other words.) Agricultural workers and others constantly outside would have plenty of this adaptation, and this look would absolutely be a cultural marker of the manual laborer.