As an adjective, 铁磁 is something like "tight" -- it describes a close friendship. It can also be a noun, in which case it just means "close friend; homeboy." 磁 can also be used on its own in the same way, though as a noun I've usually heard it as something like 磁儿. 铁 just means "solid, strong" as in 铁哥们儿.