Put simply, a timestamp records when a signature was created. Because all code signing certificates expire it is important for any software verifying a digital signature to know if the signature was created before or after the certificate expires. If you use a timestamp your signatures never expire, even when your certificate does.
kSign automatically timestamps when it signs but other code signing utilities like Microsoft's signtool.exe need to be passed a URL for a timestamp server. Comodo offers a timestamp server for free for any certificate holder, the URL is http://timestamp.comodoca.com/authenticode
For Microsoft's SignTool (4.0 and later) and others applications like Jarsigner which support RFC 3161 (Time Stamping Protocol): use the URL http://timestamp.comodoca.com/rfc3161
NOTE : For signing macros in MS Office / VBA - use the Authenticode URL (http://timestamp.comodoca.com/authenticode)