This is the 4705th most frequent Russian word.
железные
"Iron" or "made of iron."
In this sentence, 'железные' is an adjective describing attributes of the gates, implying they are made of or resemble iron.
Эти железные ворота очень прочные.
These iron gates are very sturdy.
Here, 'железные' is used metaphorically to describe rules that are strict and unyielding.
Железные правила требуют соблюдения.
Iron rules require compliance.
In this sentence, 'железные' is figuratively describing nerves as being exceptionally strong and resilient, a common idiomatic usage.