This is the 3367th most frequent Russian word.
полезные
"Useful" or "beneficial."
The word 'полезное' means 'useful' here, describing the activity of reading as beneficial.
Чтение книг — это полезное занятие.
Reading books is a useful activity.
The word 'полезны' is used to indicate that fruits are beneficial for health, showing a connection between usefulness and well-being.
Свежие фрукты очень полезны для здоровья.
Fresh fruits are very healthy for your health.
The word 'полезные' in this sentence modifies 'советы' (advice), emphasizing their practical value in the context of studying.