There can be many interpretations. The Jews were of the law. Christ made a new covenant and He fulfilled the law. He entrusted the apostles with the gospel message and since they had the power of the Holy Spirit, whatever they say would go and God would honor what they decided. Example, Paul announced the end of purification. Another example, they could now eat whatever meat at the meat market, including pork which was always unlawful with the Jews. They were no longer under the law of rituals or sacrifice, they no longer had to be circumsied. These are just a few examples of what was loosed. In other words, through the inspiration of the Holy Spirit, the apostles were given the power to loose or retain. In another verse like this was, if you forgive the sins of any, they are forgiven, if you retain the sins of any, they are retained.
