What does the green circle with a white tick mark icon mean?

VirusTotal makes use of the checkmark symbol to indicate that the given file was not detected in any way by the antivirus under consideration. We do not use the word "clean" or "innocuous" because antivirus solutions do not tell you whether a file is goodware, they just flag maliciousness.

Other symbols that may appear are:

 The antivirus under consideration detected this resource.
Timeout while running this antivirus.
Unable to process file type.