This is the 182nd most frequent Polish word.
napisał
"Napisał" means "he/she/it wrote."
In this sentence, 'napisał' is used in its basic past tense meaning to describe a completed action of writing something, in this case, a book.
On napisał książkę.
He wrote a book.
Here, 'napisał' is used in a question structure to inquire about the person who performed the action of writing the letter.
Kto napisał ten list?
Who wrote this letter?
In this context, 'napisał' is used to indicate the action of sending a written message or correspondence to the speaker, specifying the time of occurrence.