This is the 252nd most frequent Bulgarian word.
вчера
"Вчера" means "yesterday" in English.
In this case, 'вчера' is part of a reported sentence, placed to indicate the timeframe of the mentioned action.
Чух, че вчера е валяло дъжд.
I heard that yesterday it rained.
Here, 'вчера' functions as the subject of the sentence, referring directly to the preceding day.
Вчера беше прекрасен ден.
Yesterday was a wonderful day.
The word 'вчера' is used as an adverb to indicate the day preceding today.