This is the 7639th most frequent Russian word.
железный
"Iron" or "made of iron"; metaphorically, "strong" or "resolute".
In this sentence, 'железный' is used to describe something made of iron.
У нас железный мост через реку.
We have an iron bridge across the river.
Here, 'железный' is used metaphorically to describe determination and resilience.
Она обладает железной волей к победе.
She has an iron will to win.
In this instance, 'железный' is applied metaphorically to indicate robustness and unassailability.