Galatians 4:1 & 2
"What I am saying is that as long as the heir is a child, he is no different from a slave, although he owns the whole estate. He is subject to guardians and trustees until the time set by his father." NIV translation
In chapter three, we saw how we, as Christians, were all made to be sons of God through faith in Jesus Christ. This passage is talking about the period of time in a child's life before they are mature and considered to be legally of age. Today, in the USA, that is determined in many cases by the law such as legal drinking age, drivers license, etc.
During Paul's time, it was not up to the government or a law but the father of the child. Until the father decided that the son was mature enough, he was considered to be a child and, though he was a legal heir, he did not have control of anything to do with the estate. The child was still, in effect, a slave to the guardian or trustee that was put in charge of them.