This word marks an event that happened because of a previous event. In this case, the previous event is Barnabas and Saul being set apart by the Holy Spirit.
Possible meanings are that 1) "there were multiple Jewish synagogues in the city of Salamis where Barnabas and Saul preached" or 2) "Barnabas and Saul started at the synagogue at Salamis and continued to preach in all the synagogues they found while they traveled around the Island of Cyprus."