A target letter sent by the Department of Justice is a formal notification of an investigation for criminal charges.
Receiving a target letter does not necessarily mean the government is prosecuting you for a crime. It is the Department of Justice’s way of notifying you of your status, which is a requirement if the DOJ seeks to issue a subpoena for your testimony.
Target letters generally describe the nature of the criminal investigation and provide notice of constitutional rights. In addition to testifying, you may be asked to produce documents related to the investigation of a crime.