Chains
The 'chains' symbolize Paul's imprisonment for the sake of the Gospel. They represent both a physical reality and a spiritual metaphor for the trials faced in the service of Christ.
The chains highlight the theme of suffering for Christ and the advancement of the Gospel despite adversity. They demonstrate how God can use difficult circumstances to further His purposes.
In the Roman world, imprisonment was a common consequence for those who disrupted societal norms, including preaching a new faith.