The Google Click Identifier (GCLID) is a parameter passed in the URL when an ad is clicked. It helps identify the campaign and other characteristics associated with the click for tracking ads and campaign attribution. In Google Ads, this parameter is enabled by activating the auto-tagging feature. The GCLID is required for tracking “Website Conversions” in Google Ads and is also used to link data between Google Ads and Google Analytics. Additionally, it can be used with features like Offline Conversion Tracking.