In an objective genitive, the genitive noun is the object of the verbal idea contained in the noun it modifies; e.g. “the love of music” – where “music” is the object of the love. She loves the music.
In a subjective genitive, the genitive noun is the agent of the verbal idea contained in the noun modified; e.g. “the love of her father” – where the love is exercised by the father. Her father loved her.
This video explains the concept for Latin learners (the concept is the same in Greek). See also GGBB p. 113 & 116.