Master, Doctor
Noun
せんせい
先生がアンナさんに手がみをかきました。
The teacher wrote a letter to Anna.
ダリン先生のつぎの休みはいつですか？
When is Dr. Darin's next day off?
先生と生徒の恋愛についてどう思いますか？
What do you think of love between a student and a teacher?
Previous life... So we're talking about someone who has a previous life, or maybe previous life experience? When you have previous life experience, you can teach others about it, making you qualified to be some kind of teacher.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.