This is the 26th most frequent Russian word.
был
"Был" means "was" (past tense of "to be" for masculine singular).
The word 'был' indicates past tense for 'was' and agrees with masculine singular subject 'он'.
Он был в парке.
He was in the park.
The word 'был' serves as the past tense 'was' for the subject 'это', referring to a singular event.
Это был хороший день.
It was a good day.
The word 'был' functions to establish the existence or occurrence of an event, in this case 'снегопад ночью', in the past.