Generically it means: Pardon us, the offenders of your law! Count us as though we never sinned!
Specifically it can be as different as the group that prays it. Forgive the compromise we have allowed into our fellowship, forgive the dissensions and gossip we have entertained, forgive the unwillingness to deal with sin in our midst…….
Forgiveness from God is what makes Christianity unique among world religions. No other religion touts free pardon for transgression! Without this essential we are like everyone else.