This is the 2129th most frequent Russian word.
южной
"Южной" means "southern" in English.
In this sentence, 'южной' is the feminine singular instrumental form of the adjective 'южный', describing the location of the 'стороне' (side) as being 'southern'.
Этот дом расположен на южной стороне города.
This house is located on the southern side of the city.
Here, 'южной' is the feminine singular instrumental form of the adjective 'южный', indicating the region 'части' (part) as 'southern'.
Путешествие по южной части страны было увлекательным.
The journey through the southern part of the country was fascinating.
In this sentence, 'южной' is the feminine singular dative form of the adjective 'южный', describing the river 'реке' as 'southern'.