The ‘Discourage search engines from indexing this site’ box can be found in your WordPress dashboard under Settings>Reading. If it’s ticked it means that the search engines won’t list your site.
Why is it there? Typically, you would tick it while you’re building your site so that the search engines don’t see your work in progress, though it won’t stop anyone who knows your domain from looking at the site – that needs you to put your site into maintenance mode.
And when your site is ready to be made public you MUST remember to untick this box or you’ll never get found!