And because you are sons, God has sent forth the Spirit of His Son into your hearts, crying out, "Abba, Father!" (Galatians 4:6; NKJV)
Because – the reason why anything is said to be or to be done, because, since, for that, for
Sons – descendant, one of the posterity of anyone; kinship
God – theos – context – the Father
Sent forth – send out on a mission to another place (control over/directing all things) – see 4:4
Spirit of His Son – pneuma – the Holy Spirit
Into your hearts – kardia – seat and center of all physical and spiritual life
Crying out – krazo – a primary verb; properly, to "croak" (as a raven) or scream, i.e. (genitive case) to call aloud (shriek, exclaim, intreat)
In brief, the word “Abba,” as it was used by the Lord Jesus and the Holy Spirit, signifies the essence of what it means to have a personal relationship with GOD through Jesus and the Holy Spirit.