This is the 1010th most frequent Polish word.
byli
"Byli" means "they were" in English.
Here, 'byli' conforms to its role as a verb in past tense for a group characterized by masculine persons. The sentence shows their emotional state at a given moment.
Byli bardzo szczęśliwi, gdy usłyszeli te wiadomości.
They were very happy when they heard the news.
In this case, 'byli' again serves as the verb 'to be' in the past plural masculine or mixed group. It's used to describe a particular role or situation of the subjects related to the event.
Byli świadkami tego zdarzenia.
They were witnesses to the event.
The word 'byli' functions as the past tense form of 'to be' specifically for the plural masculine or mixed-gender group. The sentence indicates that the group was in a certain place (Spain) at a specific time in the past.