In short, a primary DNS is a giant address book. When you type in a url into your browser. Your browser will look up that url in the primary DNS. The primary DNS will report back and tell your browser how to get to the site you're looking for.
See not as magical as you thought. It is a giant list that will translate a name to a bunch of addresses to get you to your destination.
Well you might have thought you didn't. But actually you do. Your internet service provider typically sets this to something for you by default. When they connect you to the network they will often set you to a primary DNS that they can see your requests and control your results. They can see every site you visit, as well as all the apps and devices you have. This can be quite scary!
To combat someone tracking you, change your primary DNS. It's actually not that difficult and there are a lot of great DNS directories out there.
I've been using Cloudfare (188.8.131.52) for some time now. They encrypt your data, they protect your privacy and they tout themselves as the fastest. I haven't done much testing of speed, but I will say I get results fast.
Another common ones is Google's (184.108.40.206). However, I really don't want Google having more information about me. Can't get any more evil then giving them your network traffic too. So I highly recommend Cloudfare at this time.
Cloudfare has instructions on how to set this up. It will only take you a couple minutes to a faster more private experience. You're welcome!