This is the 15282nd most frequent Russian word.
послу
"Послу" is the dative form of "посол," meaning "ambassador" in English.
In this sentence, 'послу' is in the dative case, used to indicate the indirect object receiving the action of giving.
Я подарил книгу послу на память.
I gave the ambassador a book as a remembrance.
Although 'послу' is not present here, 'посол' in nominative case is used to refer to the ambassador performing the action of reporting.
Посол сообщил о результатах встречи.
The ambassador reported the results of the meeting.
Here, 'послу' again appears in the dative case indicating the recipient of the letter being written.