This is the 43rd most frequent Ukrainian word.
були
"Були" means "were" in English.
The word 'були' here shows the past tense of the verb 'to be' for the plural subject 'we'.
Ми були в парку вчора.
We were in the park yesterday.
The word 'були' here indicates the past tense state of 'the children' being happy, used with an adjective.
Діти були щасливі гратися разом.
The children were happy to play together.
The word 'були' is part of a subjunctive mood construction, used to describe a hypothetical past situation.