It depends. If you are using the Desktop/Download version of TurboTax, in Forms mode, red letters may indicate that data has either been overridden or is invalid (for example, a nonexistent ZIP code). See here for more information about the color coding in Forms Mode.
In order for us to better assist you, please clarify what message appears on your screen, in what section of TurboTax, and what version of TurboTax you are using.