Here the word "service" refers to Paul and his companions bringing the contribution to the believers in Jerusalem. AT: "For our carrying out this service for the believers in Jerusalem" (See: [[rc://en/ta/man/translate/figs-explicit]])
Paul speaks of the Corinthian believers' act of service as if it were an object that could multiply itself into many objects. AT: "It also causes many deeds for which people will thank God" (See: [[rc://en/ta/man/translate/figs-metaphor]])
"for his gift, which words cannot describe." Possible meanings are 1) that this gift refers to "the very great grace" that God has given to the Corinthians, which has led them to be so generous or 2) that this gift refers to Jesus Christ, whom God gave to all believers.