Jacoby Brissett is the only quarterback on the roster for the Colts. They decided to cut Phillip Walker, and Chad Kelly is suspended for the first two weeks of the season. Because of that suspension, the Colts need to sign a backup quarterback.
They have been brining in a slew of veterans in the past week. Since they put Walker on the practice squad, it seems likely that they will sign someone for just the first couple of weeks and then let them walk. Kelly would then be then be the backup for the rest of the season.