Teacher, Sensei
noun
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. 先生 is often used for school teachers, but it's also used to show respect to doctors, lawyers, authors, and other respected experts. It's used as a standalone word or a name ender like トム先生.
せんせい
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.
先生は「おはよう」といった。
The teacher said "good morning."
あの女の人は、田中先生です。
That woman is Tanaka-sensei.
コウイチは、わたしの人生の先生だ。
Koichi is my teacher in life.