This is mainly used when we use Apex Metadata API.

Used to Deploy Metadata from Non-Certified Package Versions via Apex. By enabling this setting, ISVs can test managed packages that aren’t yet certified, and enterprises can use managed packages to test or update their apps. But as long as it’s turned off for your org, you can rest assured that no uncertified package can modify your org.

1. Go to Develop –> Apex Settings.

2. Enable Deploy Metadata from Non-Certified Package Versions via Apex and click “Save” button.