This is the 2447th most frequent Russian word.
полезно
"Полезно" means "useful" or "beneficial."
In this sentence, 'полезно' is used to describe something that is beneficial or advantageous for improving intellectual capacity.
Много читать полезно для ума.
Reading a lot is beneficial for the mind.
Here, 'полезны' (the plural form of the adjective) describes vitamins as being beneficial for maintaining good health.
Витамины полезны для здоровья.
Vitamins are good for health.
This usage of 'полезно' expresses that paying attention to details is something advantageous and recommended.