This string, proposed by EICAR (the European Institute for Computer Anti-Virus Research), is treated like a virus by most security and antivirus programs. The reason for this is that it's a standard test to see if your antivirus protection is working.
The file itself is harmless -- all it does is print "EICAR-STANDARD-ANTIVIRUS-TEST-FILE!" on your screen. But it's a null payload that you can use to demonstrate sound security measures without risking actual harm. Paste it into a file called "eicar.com" and see whether your antivirus software detects it.