There are many uses for DNS resolution and IP blockers, but the most prominent uses are getting rid of ads in both browsers and apps, and protection against malware sites. Please note that "Block it!" does not block anything by default; it is your choice what you want to filter, and yours alone. There is a live log of all DNS resolution requests that you can use to determine what you want to block; you'll be surprised what your phone accesses all the time. You can also use blacklists available on the Internet (in "/etc/hosts" format, Snort format, and many others). To get you started, just click "add all lists" in the welcome dialog; you can always remove lists again later.
"Block it!" uses the VpnService API and therefore does not need root access. When you start the app for the first time since reboot you will be asked by Android whether you want to permit the VPN service. Rest assured that we do not redirect any traffic; all we do is catch DNS requests and filter them before we forward them to your device's configured DNS servers, and redirect blacklisted IP traffic to the VPN interface where it is simply dropped. When "Block it!" is active, a key symbol will be displayed in the status bar -- Android's way of reminding you that you are using a VPN service. We cannot do anything about it.
Please note that you cannot run "Block it!" while you are using any VPN service. This includes "Your Freedom". However, "Your Freedom" includes blocking features too so you don't need to use both at the same time.
"Block it!" is free and we do not pester you with ads. If you find that it makes your life easier please click on the "thumbs up!" symbol in the action bar to learn how you can help us.