Used between couples in a relationship to address each other. Normally used by younger couples (old generation does not use this phrase), and can be between unmarried or married couples.
It can be addressed to both men and women.
In Korea, the word literally means "Self" - so you are literally calling the other person as yourself. So that implies the other person is your self, your other half, your soulmate, you...
Jagiya, call me~!
I love you, jagiya~!