This information applies to Windows Internet Explorer 7 and Windows Internet Explorer 8.
A digital signature is an electronic security mark that can be added to files. It allows you to verify the publisher of a file and helps verify that the file has not changed since it was digitally signed.
If the file does not have a valid digital signature, there is no way for you to be sure that the file is actually from the source it claims to be from, or that it has not been tampered with (possibly by a virus) since it was published. It's safer to avoid opening the file unless you know for certain who created it and whether the contents are safe to open.
Even a valid digital signature does not verify that the contents of the file are harmless. You must decide if you should trust the contents of the file based on the identity of the publisher and where you are downloading the file from.