Gclid is a globally unique tracking parameter (Google Click Identifier) used by Google for tracking purposes. for example, to pass information back and forth between Google AdWords and Google Analytics. If you enable URL auto tagging in Google AdWords, Google will append a unique GCLID parameter on your destination URLs at run-time. Because it is a redirect, you won't see the parameters on your ad words text ad destination url's, but it will show up in your Web server log files.
In adCore you can use the GCLID to track your offline conversions and upload them back in to AdWords. That way, if a sale starts online (e.g. your customer left a lead on your website), you can still upload it to AdWords through adCore and make sure you are tracking your conversions accurately.