This is the 1915th most frequent Russian word.
пришли
"Пришли" means "came" or "arrived" (past tense, plural).
The word "пришли" here is used to express that the subjects completed the action of arriving at a destination, specifically the library.
Они пришли в библиотеку рано утром.
They came to the library early in the morning.
In this sentence, "пришли" is used to indicate the arrival of letters, showing a completed action of delivery.
Письма пришли вчера вечером.
The letters arrived yesterday evening.
Here, "пришли" is used in a question to inquire about the time the action of arriving home took place.