The WiiM offers an override to 8.8.8.8, which is the Google DNS. In general, DNS is dictated by your ISP, so for example, if you have Comcast/Xfinity, you’re directed to their DNS. The DNS is important in that it translates the domain name requested into the IP address needed to get you there. Some DNS servers are slower and some faster, some are less or more secure, so WiiM gives you the option to override your ISP DNS for the Google DNS, which may or may not give you a better experience.
I’ve read that some users have had issues with this feature enabled so you can just disable or you can try it out and see if it makes a difference. More technically advanced users are probably bypassing their ISP DNS in their router for options from Google or other options, in my case, I use Cloudflare’s 1.1.1.1. DNS.