여기 = here = next to me (to the person who speaks) 거기 = there = next to you (to the addressee) 저기 = over there = far from both speaker and adressee in addition, 거기 can reffer to a place which both speaker and addresse know or which was previously mentioned in the conversation. like in the sentences - 'Do you remember the bar we went to on the last Friday? I've been there today' - you or your addresse both are not there at the moment of speaking, but you will still use 거기 as you refer to the place you both know and are talking about. also, you can use 저기요 to politely draw attention of someone like stranger on the street, or waitress (comparing to the English "Hey, over there!", 저기요 is not rude at all).

Similar to the trinity of the demonstrativa 이, 그, 저 you have for a trinity localisation 여기, 거기, 저기 ~ here (this place here), there (the place there), over there (thus place over there) ..thats how I understand it XD

거기 : 앞에서 이미 이야기하였거나 듣는 이가 생각하고 있는 장소를 가리키는 말. ex) "네가 말했던 장소에 가니까 거기에 물건이 있더라" 저기 : 말하는 이와 듣는 이로부터 멀리 떨어져 있는 곳을 가리키는 말 ex) 네가 아까 찾던 사람 "저기" 가니까 있더라.