This is the 12082nd most frequent Russian word.
идущему
"Идущему" - "to the one going" or "to the one who is walking."
In this sentence, 'идущему' is a present active participle in dative case, describing 'human' (человеку), indicating an ongoing action of walking.
Я подошёл к идущему человеку.
I approached the person who was walking.
Here, 'идущему' is in dative case, used to describe the child (ребёнку) who is walking, as part of a command or request.
Помогите идущему ребёнку.
Help the child who is walking.
In this context, 'идущему' serves as a descriptive participle in dative case, modifying 'student' (студенту) to describe their activity while receiving the book.