Acts 15:1 & 2

"Some men came down from Judea to Antioch and were teaching the brothers: 'Unless you are circumcised, according to the custom taught by Moses, you cannot be saved.' This brought Paul and Barnabas into sharp dispute and debate with them. So Paul and Barnabas were appointed, along with some other believers, to go up to Jerusalem to see the apostles and elders about this question." NIV translation

So, while Paul and Barnabas were at their "home church" in Antioch, some men tried to tell the people that they must be circumcised to be saved.

Paul and Barnabas knew that it was wrong and it caused a dispute there. Instead of simply standing on the teaching of Jesus and their experience from their missionary journey, a group of men were sent to Jerusalem to get an answer.

Acts 15:3 & 4

"The church sent them on their way, and as they traveled through Phoenicia and Samaria, they told how the Gentiles had been converted. When they came to Jerusalem, they were welcomed by the church and the apostles and elders, to whom they reported everything God had done through them." NIV translation

On their way to Jerusalem, they explained how God had allowed them to share the gospel of Jesus with the Gentiles on their previous journey. When they came to Jerusalem, they reported the same thing to the church there and it's leaders.

Acts 15:5 & 6

"Then some of the believers who belonged to the party of the Pharisees stood up and said, 'The Gentiles must be circumcised and required to obey the law of Moses.' The apostles and elders met to consider this question." NIV translation

So, some of the Pharisees stated that the Gentiles must obey the law of Moses to be saved and the leaders of the "church" in Jerusalem met to discuss the matter. These people were trying to add their rules to Jesus' sacrifice instead of realizing that salvation is truly a gift from God by faith alone. The leaders of the church in Jerusalem did not want to make anyone mad so they did not set them straight right away. They had a meeting to discuss the question.