In this verse Paul says that not to think like a slave but one who loves and cares for another (figuratively, Christian sonship in respect to God). “Abba” is an Aramaic word for father, used by Jesus and Paul to address God in a more intimate and personal way.
It reads in Romans 8:15-16 NIV: The Spirit you received does not make you slaves, so that you live in fear again; rather, the Spirit you received brought about your adoption to sonship. And by him we cry, “Abba, Father.” The Spirit himself testifies with our spirit that we are God’s children.