Professions let you gather resources and craft your own items, from gear to consumables.
Gathering professions
Lumberjack, Miner, Farmer, Fisherman and Trapper are examples of gathering professions: they let you collect raw materials directly from the map.
Crafting professions
With those raw materials, crafting professions (Weaponsmith, Jeweler, Tailor, Baker, among others) let you craft items using specific recipes.
Leveling up your profession
Every time you gather or craft something, you gain profession experience. The higher the level, the more advanced resources you can gather or craft.