This is the 8287th most frequent Korean word.
n에
"n에" means "to/in/at/on n" depending on the context.
The particle "에" is used to indicate the time at which the action of meeting occurred, attaching to "월요일" (Monday).
나는 월요일에 친구를 만났어요.
I met my friend on Monday.
Here, "에" specifies the location where the pencil is found, attaching to "책상" (desk).
책상에 연필이 있어요.
There is a pencil on the desk.
In this sentence, "에" marks the destination or location of the subject's living, attached to "한국" (Korea).