What does it mean when you get an error message that says "Illegal Characters Detected" when uploading documents or images?

What does it mean when you get an error message that says "Illegal Characters Detected" when uploading documents or images?

The error message is referring to the name of the file you are uploading. There are illegal Characters/Special Characters on the file name. To resolve this, you just need to your files and rename the actual file you are trying to upload.