Blokada provides a list of pre-defined DNS servers you can choose from.
The acronym DNS stands for Domain Name System. Your device communicates with DNS servers, which are like road signs, helping to find the places on the Internet you want to visit. For example, when you want to open google.com, the DNS server will tell your device which IP addresses to connect to.
Your Internet Service Provider (the operator of your WiFi or mobile network) has its own DNS servers, and your device will use them by default. With Blokada however, you may switch to one of reputable and trustworthy DNS servers. They should be used if you are suspecting that your ISP collects logs about your internet usage, you are afraid of some privacy risk or if some websites or services are blocked for you.
Keep in mind that there is no "best" choice, everything depends on your preferences and location, therefore it's a good idea to do a little research about your choice.
Changing DNS is a free feature in Blokada, and you may use it without the Blokada Plus / Blokada Tunnel subscription.
Also if you are interested in encrypted DNS, Blokada 5 (currently available on iOS only, coming to Android next) supports DNS over HTTPS out of the box. In fact, all servers listed in the app are DoH enabled.