This attribute contains an adult status and affects in which countries your product ads are shown.
You have to use the attribute if you want to submit items that are considered 'not appropriate for children' and want to show it at the item level.
The 'adult' attribute is an addition to the option in the General settings tab of your Merchant Center account. This option allows to select that your product data may contain products that are considered 'not appropriate for children'. If you use the 'adult' attribute you inform your customers that some of your items or your entire website are only appropriate for an 'adult' audience. You must ensure that the landing page that contains family-safe products is free from adult content.