The exclusion parameter is a URL parameter that the natural search tag uses to determine if a visit originates from a paid search ad. When a page loads on your site, the Natural Search tag checks for this parameter. If the parameter is in the URL, the tag reports the visit as originating from a paid search click.
If you haven't already added the parameter as part of some other tracking solution, you'll need to add the parameter to all of your keyword landing page URLs.
The exclusion parameter in accounts that support parallel tracking
If you use an exclusion parameter other than gclid, your accounts that support parallel tracking must define the parameter either in a Final URL Suffix or directly in each final URL. Because of parallel tracking, URL parameters that are defined in URL templates may not available to your landing pages.