ID display at the top of each page

I am new to WordPress. I have constructed a blog; I’m happy with its appearance.

I use a “top menu” and a “main menu” structure. I just noticed that in the upper left corner, just above the top menu, there appears some sort of ID, consisting of 2 letters, hyphen, 8 digits, hyphen, 1 more digit, like so:

AA-12345678-1

When I copy this ID (or whatever it is) and perform a Google search, my site instantly shows up in the search result. I don’t know what the purpose of this number is, whether it’s good or bad to have it where it is. I do know that I don’t want this number displayed, but I have been unable to find information on what it is and how to get rid of it.

Hopefully someone here will be able to help. Thanks.