Indeed it is not very clear on a user point of view. Here how to do it :

• You go to the Google Search Console

• You create a ressource corresponding to your website

• You now need to validate your ressource, choosing on of the many methods proposed. Google needs it to verify you are the owner of the ressource
  

• You choose the webpage verification method : it asks you to create a webpage at the root of the domain, such as : google123456789123.html

• You give the name of this web page in the code fields in the Odoo settings page above

• You save this setting in Odoo, it will create this web page for you.

• and you ask Google to validate the ressource.
  

• Then you can disable the web page on the Odoo side by unchecking the setting.