This is the 4698th most frequent Russian word.
елки
"Ёлки" means "Christmas trees" or "fir trees" in English.
In this sentence, 'ёлки' is used to refer to fir trees in their literal sense.
Я вчера гулял в парке и видел красивые высокие ёлки.
Yesterday, I walked in the park and saw beautiful tall fir trees.
Here, 'ёлки зелёные' is an idiomatic expression used as an exclamation to show surprise or confusion.
Ой, ёлки зелёные, не ожидал такого поворота событий!
Oh, green fir trees, I didn't expect such a turn of events!
In this sentence, 'ёлки' refers to Christmas trees, a common decoration for the holiday season.