This is the 27th most frequent Slovak word.
ktorí
"ktorí" means "who" or "which" (masculine plural, relative pronoun).
The word 'ktorí' is used here to refer to 'žiaci' (students), providing additional information about them, specifically identifying the sub-group of students who prepared.
Žiaci, ktorí sa pripravovali, uspeli na skúške.
The students, who prepared, succeeded in the exam.
Here, 'ktorí' describes 'ľudí' (people), identifying them as the group who needed help.
Videl som ľudí, ktorí potrebovali pomoc.
I saw people who needed help.
In this sentence, 'ktorí' refers to 'známych' (acquaintances), giving more detail about the acquaintances by specifying their location.