This is the 17345th most frequent Korean word.
선생님을
"선생님을" means "teacher" in the accusative form (as the object of a sentence).
Here, '선생님을' is the object of the verb '만났습니다' (met). It is used in the accusative case to indicate the teacher as the person encountered.
선생님을 만났습니다.
I met the teacher.
Here, '선생님을' is used as the object of '존경합니다' (respect). It marks the teacher as the entity being respected.
나는 선생님을 아주 존경합니다.
I deeply respect the teacher.
In this sentence, '선생님을' is part of the phrase '선생님을 위해' (for the teacher), indicating the purpose or recipient of the action performed by the speaker.