This is the 36th most frequent Hebrew word.
היו
"היו" means "were" or "have been" in English.
Here, 'היו' is used as part of the past tense verb phrase indicating possession or existence of dogs in the past.
היו לי שלושה כלבים כשהייתי ילד.
I had three dogs when I was a child.
In this context, 'היו' serves to express the existence of reasons in the past tense.
לא היו סיבות לדאגה, הכל היה בסדר.
There were no reasons to worry, everything was fine.
Here, 'היו' is employed in a conditional past construction to express a hypothetical scenario.