Explain Callout Extensions in Google Adwords?

Callout extensions can be used with other ad formats, like sitelinks, and are created, scheduled and managed within the Ad Extensions tab. You can add callouts at the account, campaign or ad group level, allowing you to use general information at the account or campaign level and specific callouts for each ad group.

You can add callouts at the account, campaign, or ad group level. You choose where to add them, create the callout text, and schedule when you'd like them to appear. The order of your callouts, their length, and how they perform factor into how many callouts appear, and whether a callout will show for your ad.