When an ad is created, or it's content, geo targeting, or traffic type is edited, it will undergo review. During this time, the ad will be Pending. A pending ad will be shown as greyed out, and if you hover over it, it will state that it is pending.
When your ad is approved you should receive an email to inform you that the ad has been approved. In addition, the Pause/Resume button will now show as green:
If your ad is declined, you will see a red stop sign . You can click on this for a breakdown on why the ad was declined.