Running ads immediately means that all new ads that are eligible to show on your site will be allowed to appear on your pages automatically. You can then review ads and take action on them by visiting the Shown ads tab in the Ad Review Centre.
The alternative to running ads immediately is holding ads for review. When you choose to hold ads, you'll have 24 hours to take action on new ads before they are automatically allowed to run on your sites. Holding ads may have a negative revenue impact, because ads that are on hold won't compete in the auction on your site and therefore won't drive up potential earnings on your site.
When you first opt in to the Ad Review Centre, the default option is to run ads immediately.