A meta verification snippet is a meta tag for verifying that you are the owner of a website. This is used for example by Google Search Console. By adding the meta verification snippet in the correct location, the third party service would be able to detect the snippet and confirm the website ownership.
To add a meta verification snippet to your site:
Click on the CogwheelIcon then go to Settings.
A panel open up with 4 different tabs, select the tab called General.
Under the section, Website, you can find a field named Meta Verification Snippet. This is the area for adding your meta verification snippet provided by the third party service.
Please note that some third party services, like Google would check if your verification method is still valid regularly. It is recommended to keep the meta verification snippet in your admin panel even though the verification process is complete.