The Iron Rune, known as the Pavise Shield in the Japanese version, is an item that makes its user immune to critical attacks when held, which can come in handy for units with terrible luck. It is obtained from either Linus or Lloyd in the Cog of Destiny chapter. It is equivalent to the Hoplon Guard in Fire Emblem: The Sacred Stones, which is obtainable from Caellach in Chapter 15 and again in the Tower of Valni when you get him.
A 'pavis' is a large shield used in the Middle Ages. The name derives from the Milanese city of Pavia in northern Italy.