184.108.40.206 is the recursive DNS operated by Google. By configuring this, your DNS queries will go to the Google DNS server rather than your ISP-provided DNS server.
DNS is encrypted so that your ISP may monitor it. So if you are using Google DNS servers, there is also a chance that Google also monitors it. But Google gives more focus to user's privacy, so it's better to use Google DNS servers rather than using ISP's DNS servers.