This is the 21st most frequent Croatian word.
bio
"Bio" in Croatian means "was" (masculine singular, past tense of "to be").
Here, 'bio' is the masculine singular past tense form of the verb 'biti' (to be), used to indicate a state or action in the past.
On je bio u parku jučer.
He was in the park yesterday.
In this sentence, 'bio' serves the same grammatical role as the past tense of 'biti', agreeing in gender and number with the subject.
Boja zidova je bio svijetlo plav.
The color of the walls was light blue.
Here, 'bio' is again the past tense of 'biti', and it is used to establish a historical fact about a person.