Fodder refers to any sort of gear that is collected for the sole purpose of enchanting it. Fodder is usually top tier hit or tank gear. Fodder may already be enchanted, or it can be devoid of enchantments. The only hard fast rule about fodder is that it is at a level such that after it is enchanted, it is still wearable.
For example, a level 53 Shield Of Heroes can be fodder, since after it is enchanted, it will become level 54, which is still useable by heroes. It may still be considered fodder even if it is already enchanted.