A more thorough explanation:

An unindicted coconspirator is a person who is identified by law enforcement as a member of a criminal conspiracy but has not been named in the indictment of the fellow conspirator. This means that the person has not been formally charged with a crime, but they are believed to have participated in the illegal activity.

Prosecutors often name someone an unindicted coconspirator because any statement that the person has made in the course and furtherance of the conspiracy is admissible against the indicted defendants. This means that even though the unindicted coconspirator has not been charged, their statements can be used as evidence in court against the other defendants.

For example, if a group of people are accused of planning a bank robbery, and one of them is identified as an unindicted coconspirator, any statements that person made about the robbery plan can be used as evidence against the other defendants. Even though the unindicted coconspirator has not been charged, their involvement in the conspiracy can still be proven in court.