are there any feats or rules that would allow me to take a hit for one of my allies so that i will take the damage instead of them?
The Stalwart Defender PrC from the APG has an ability you can choose that lets you take a hit intended for an ally.
There is also a feat: In Harm's Way.
There is also a feat: In Harm's Way.
how does the feat work? what are the prerequisites?
The prerequisites are just another feat, Bodyguard. When you use the Aid another action to raise someone's AC you can take an immediate action to suffer the attack yourself. Bodyguard has Combat Reflexes as a prerequisite.
Order of the Lion Cavaliers also get an ability at level 15 that lets them redirect an attack on an adjacent ally to themselves as an immediate action.