The "pending" checkbox for an external link tells IFDB that the file isn't available for download yet, but will be soon. If you check this box, IFDB will do two things:
This option is especially useful with the IF Archive, because files uploaded to the Archive are not available for download until they're reviewed by administrators. This process usually takes a few days. You could just wait to add the download link until the file appears on the Archive, but the "pending" option saves you that extra editing step. Just check the box, and IFDB will monitor the Archive for you, updating the link as soon as it's working.