They are Jews who opposed the Gentiles in the faith.
Another answer:More specifically, biblical judaizers (especially from among the Pharisees) insisted that converted Gentiles needed to be circumcised and keep the law of Moses. Acts 15 deals extensively with the matter, and with its resolution.I assume you are asking where this phrase originates. The Apostle Paul was taking about the Judiazers, who wanted new Christians to be circumcised. He believed their philosophy was wrong because it would put believers back under a legal system that had failed. He wrote in Galatians 5:12, 6:12-13, 6:15 "I wish the people who are bothering you would go the whole way and castrate themselves...It is those who want to look good outwardly who are tying to get you to be circumcised."
The Church has always been an organization that spreads the Gospel of Salvation. However, not everyone understands the message properly all the time and sometimes go astray in the thoughts and beliefs. Those who err are called heretics and the Church has always attempted to correct the errors in their beliefs. It is not, and never has been, a control issue as errant beliefs can jeopardize the soul and the Church must attempt to correct those errors or she herself is in error. So, when a grave heresy is or was detected, the Church attempts to correct the error via discussions regarding doctrine and the reasons that the issue is wrong. If the persons are not recalcitrant, like the Arians who denied the divinity of Christ, they are not compelled to believe the truth they are urged to do so for their sake. However, they are free to leave the Church and jeopardize themselves. The first council to ever deal with errors in beliefs was the Council of Jerusalem that dealt with Judiazers (see Acts 15).