This is the 29th most frequent German word.
sein
"Sein" means "to be" in English.
In this sentence, "sein" is used as the main verb indicating a state of being.
Er ist ein Lehrer.
He is a teacher.
In this sentence, "sein" is used as an infinitive verb following a modal verb, indicating presence.
Wir werden später dort sein.
We will be there later.
In this sentence, "sein" is a possessive pronoun indicating ownership.