This is the 17786th most frequent Russian word.
умному
"To the smart one" or "to the clever one."
The word 'умному' is in the dative case, agreeing with the noun 'человеку' ('person'). It describes the attribute of the person being referenced.
Умному человеку не трудно учиться.
For an intelligent person, learning is not difficult.
Here, 'умному' is used to describe the student ('ученику'), functioning as an adjective in the dative case, showing to whom the explanation should be given.
Это объяснение надо давать умному ученику.
This explanation should be given to the intelligent student.
In this sentence, 'умному' also functions as an adjective in the dative case, indicating the recipient of the advice ('совет').