That word "Abba" simply means "Daddy".
My oldest daughter quite frequently tells other childen that my name is "Pastor Tim" and that they have to call me "Pastor Tim", but that she gets to call me "Daddy" because it is a special name. I like that! "Daddy" is a special name that only my 3 daughters get to call me.
Romans 8:15 tells us that as Christians we get to call God, "Daddy". What a special name. As Christians we should feel such a powerful relational connection with God that we can confidently call Him “Daddy” and know that He calls us His child.