Barrier protected steel is coated with a material that prevents moisture and oxygen from coming into contact with the base metal. The absence of moisture and oxygen prevents the steel from corroding.
In a metal-coated context, coatings consist of inherently corrosion-resistant metals such as Aluminium, Tin, Gold, and so forth.
The advantages of barrier protection are that these coatings are long-lasting and resistant to temperature change, abrasion, erosion, UV and are often electrically conductive. On the downside, if the coating is scratched or cracked localized corrosion will occur.