Usually, if someone is disciplined or reproved, they may have been involved in some type of misconduct, or shown a habit of perhaps continuing in a sin. The congregation would need to be cautious in associating with that person so as not to allow themselves to come under their influence in committing a wrong action. If a person does not accept reproof or continues in misconduct, they can be disfellowshipped. However, others in the forum may be able to explain more clearly. A reason for marking is listed below in 2 Thess 3:13-15:
2 Thess 3:13 For YOUR part, brothers, do not give up in doing right. 14 But if anyone is not obedient to our word through this letter, keep this one marked, stop associating with him, that he may become ashamed. 15 And yet do not be considering him as an enemy, but continue admonishing him as a brother.