Well, it depends on a sentence. If the sentence is "저기에 나무가 있잖아", it means "There is a tree". However, if it comes alone like "있잖아, 혜진이 미국갔데", it means "You know what, Hyejin has gone to the United States". So, if "있잖아" comes alone, it means "You know, You know what, Guess what". And if "있잖아" comes as a verb on end of a sentence, it means "is or are" (Kind of hard to translate in a word)