This is the 8749th most frequent Russian word.
пили
"Пили" is a form of the verb "пить," meaning "drank" in English (past tense, plural).
In this sentence, "пили" is used as the past tense form of the verb "пить," which means "to drink."
Мы пили чай вчера вечером.
We drank tea yesterday evening.
Here, "пили" also serves as the past tense form of "пить," but specifically indicates a continuous action happening in the past.
Пока мы пили кофе, шел дождь.
While we were drinking coffee, it was raining.
In this example, "пили" emphasizes the considerable amount of wine consumed in the past action.