How do I interpret 铁饭碗 concisely?
I think translating 铁饭碗 as "guaranteed lifetime employment" gets the point across quite well. There's also a phrase, "cradle-to-grave socialism" -- it's a little more political than guaranteed lifetime employment, but it accurately reflects the use of the word in Chinese history.

Neither phrase has the English resonance of the Chinese original, but I think both are acceptable choices if you are seeking a short explanation.