Some translations set poetry farther to the right than the rest of the text to show that it is poetry. The ULB does this with the poetry that is quoted from the Old Testament in 15:16-17.
The meeting that Luke describes in this chapter is commonly called the "Jerusalem Council." This was a time when many church leaders got together to decide if believers needed to obey the whole law of Moses.
Some believers wanted the Gentiles to be circumcised because God had told Abraham and Moses that everyone who wanted to belong to him had to be circumcised and that this was "an everlasting covenant." But Paul and Barnabas had seen God give uncircumcised Gentiles the gift of the Holy Spirit, so they did not want the Gentiles to be circumcised. Both groups went to Jerusalem to have the church leaders decide what they should do.