Salt made by drying seawater, sold coarse or fine. Untreated sea salt has no iodine, unlike iodised table salt, and much ordinary fine salt is sea salt too, though the bag rarely says so.
海盐 also names Haiyan County in Zhejiang, a historically important salt-producing area. Capitalization in pinyin (Hǎiyán vs hǎiyán) distinguishes the place name from the common noun.