Hola Yeppy! 😀
The sentence means: “Jaime went to the library to get the book because he has to study hard at night for tomorrow’s exam.”
In this sentence we use “por” because we are using a movement verb. The structure goes like this:
MOVEMENT VERB + A + POR + SOMETHING (to go, to come, to return…)
Ej:
– Voy a por el libro – I’m going to get my book
– Quiero ir a por el niño a las 3 – I want to go to pick up the kid at 3
Hope that helps!