This is the 4356th most frequent Arabic word.
نادر
Rare.
Here, the word 'نادر' is used as an adjective to describe an object (stone) as 'rare'.
هذا حجر نادر للغاية.
This is a very rare stone.
In this sentence, 'نادر' is part of an expression to describe an infrequent occurrence or event (seeing a double rainbow).
من النادر أن أرى قوس قزح مزدوج.
It is rare that I see a double rainbow.
In this instance, 'نادر' is used as a proper noun, specifically a personal name, which is a common usage in Arabic.