This is the 30th most frequent Russian word.
были
"Были" means "were" (past tense of "to be" for plural subjects).
Here, "были" is the past tense plural form of the verb "быть" (to be), used to indicate past presence in a location.
Вчера мы были в парке.
Yesterday, we were in the park.
Here, "были" is the past tense plural form of "быть," describing a state of existence or status.
Они всегда были известными художниками.
They have always been famous artists.
In this sentence, "были" is used as a conditional past tense form of "быть" in a hypothetical clause.