From The Gospel of Matthew by Curtis Mitch and Edward Sri, commenting on Matthew 18:18:
In ancient Judaism, the terms to bind and loose were associated with the authority to teach and to grant or withhold forgiveness of sin. Significantly for this passage, the terms also denoted juridical authority to include or exclude persons from the community of faith. While this authority was given to Peter uniquely in 16:19, it is now bestowed on the disciples as a whole. In this context, it refers to their authority to make decisions regarding the status of unrepentant sinners in the Christian community.