Rust is the common term for steel or iron which has been oxidized, a chemical change caused by exposure to oxygen, frequently the oxygen in water. Rust appears as a brown or orange discolouration which can be scrubbed off. This discolouration can stain and damage fabric.
The best defense against rust was (and is) keeping water from contacting the metal. In period, most metal armour was painted or left covered in forgeblack, rather than being polished. If armour was polished, it needed frequent scouring and oiling to maintain it.