A more thorough explanation:

A lineup is a procedure used in criminal investigations where a suspect, who is already in custody, is placed among a group of people who look similar to the suspect. The witness is then asked to identify the perpetrator of the crime. This is a test of the witness's ability to identify the suspect.

According to the Wade-Gilbert Rule, which was created by the United States v. Wade and Gilbert v. California cases, post-indictment lineups are a critical stage of the criminal prosecution. This means that the defendant has the right to have their lawyer present during the lineup. If the defendant is denied their right to counsel, then the lineup is inadmissible.

For example, if a witness sees a person commit a crime and the police arrest a suspect, they may conduct a lineup to see if the witness can identify the suspect as the perpetrator. If the lineup is conducted after the suspect has been indicted, the defendant has the right to have their lawyer present.