Lyric discussion by Outernationalist 

Cover art for Iron Lion Zion lyrics by Bob Marley and the Wailers

Both muffinhead and Goldrocker are right. I think it's a reference to rasta beliefs (Zion is Ethiopia, the 'promised land', while the Lion is the Lion of Judah, Haile Selassie I), but it's also a reference to his assasination attempt, he had to "run like a fugitive" by exiling from Jamaica, to save his life as a rebel of Jamaica's political system.

I think the "I'm gonna be Iron like a Lion in Zion" also kinda means "I'm going to be brave and worthy", like Haile Selassie was. Marley is encouraging himself to be brave and keep forward. I think.