This is the 57th most frequent Korean word.
에
A particle indicating location, time, or direction (to, at, on, etc.).
In this sentence, '에' is a particle indicating destination.
학교에 갑니다.
I go to school.
Here, '에' is used to denote a specific point in time.
아침 8시에 일어났어요.
I woke up at 8 in the morning.
In this case, '에' is part of '안에', indicating location.