A more thorough explanation:

An unindicted conspirator is a person who is identified by law enforcement as a member of a criminal conspiracy but has not been charged with a crime. They are also known as unindicted co-conspirators.

For example, if a group of people are charged with drug trafficking, but one person who was involved in the conspiracy is not charged, they may be referred to as an unindicted conspirator. This person may have provided information to law enforcement or may not have enough evidence against them to charge them with a crime.

Prosecutors may name someone an unindicted co-conspirator because any statements they made in furtherance of the conspiracy can be used as evidence against the indicted defendants. This means that even though they are not charged, their involvement in the conspiracy can still be used in court.