This is the 573rd most frequent Croatian word.
ući
"Ući" means "to enter."
In this sentence, 'ući' means to physically enter into a space, like a room, indicating movement into a specified area.
On je odlučio ući u učionicu.
He decided to enter the classroom.
Here, 'ući' is used metaphorically to mean delving into a subject or topic.
Moramo ući u detalje ove situacije.
We have to go into the details of this situation.
This usage of 'ući' again refers to physically entering a building but with a conditional restriction.