Originally Posted by daniellamp
his phone has been flashed with unauthorized software and is locked
The message means just what it says. The phone has detected that you installed or changed some system files that it doesn't like. And is exhorting you to go to a service center and let them return your phone to 100% stock.
We would know a lot more and could have given you a lot better advice had you provided basic and critical information such as which S5 model you have and exactly which files you tried to write to your phone. Since you didn't provide those crucial details, we have no idea what is really going on. Presumably you wrote the wrong firmware to the phone or botched a rooting process.
Without any details, all I could suggest is that you use the Odin software to write a 100% stock firmware file the phone, followed by a factory data reset. This will remove any user files or changes and should resolve the error message. Then you can redo whatever you were trying to do.. root, update, etc.