Sometimes services like Google, Yandex, Bing, ... ask you to insert a piece of code into your website.
Inside the Site Settings, the button at the top right, you will see "Ownership verification".
Select the service you require, read the instructions, and paste the code.
Publish your site for changes to take effect.
Watch the tutorial video:
How to Verify Your Site with Google & Submit Your Sitemap
Note: Verifying ownership helps search engines to know your website exists. It will not improve your visibility in search results.
Learn how to make your website appear in more search results from our Step-by-Step Guide to improving your SEO. It's the first link below.