This is the 17936th most frequent Russian word.
учителю
"Учителю" means "to the teacher" in English.
In this sentence, "учителю" is used in the dative case to indicate gratefulness toward the teacher.
Я всегда благодарен учителю за его уроки.
I am always grateful to the teacher for his lessons.
Here, "учителю" in the dative case shows the recipient of the book as the teacher.
Книга, которую я купил, предназначена учителю.
The book I bought is intended for the teacher.
The word "учителю" in this example illustrates the indirect object that the letter was delivered to.