Salted butter is butter to which salt has been added. Typically, manufacturers will add 1/4 teaspoon of salt per stick (1/4 pound) of butter.
Salted butter is typically used for table butter, when the butter will be applied directly to foods. It is not recommended for baking applications, which typically will specify how much butter the baked good requires, because of variability in how much salt is added to salted butter.
Contrast unsalted butter.