DNS, or Domain Name System, is a server that translates domain names like apple.com to IP addresses, a numerical sequence of numbers that can locate servers on the internet. Your ISP has its own DNS server that is used by default on all your iOS devices, but you can also point it to a different DNS server.
Why would you want to change the default DNS server for the purposes of reading digital content?
Most ISP's DNS servers aren't fast enough, and because DNS lookups are performed very often this could impact your reading experience. Furthermore, if the DNS security is particularly strict this can actually restrict the content that you are able to see, let alone read and download, so it can be very beneficial.
YUDU recommend changing to the Google DNS service, and here are the instructions on how to do this.
Note: screenshots and instructions taken from iOS 8.3 on iPhone 6 Plus. Most devices are similar but earlier versions of the operating system may differ slightly.
To begin, select Settings
From the Settings menu select the Wi-Fi option
Press the i button next to the Wi-Fi connection (most likely this will be at the top if you are already connected)
You should now see a DNS field with values already entered, however press onto the values and remove them. Now enter 18.104.22.168, 22.214.171.124 (a comma separating the two with a space after the comma) and press enter.
You have finished changing the DNS server for your iOS device. This should be easily replicated across Apple tablets and phones, but of course this can apply to other electronic devices.