Moblock (moblock-deb) is a so called traffic blocker. It prevents connections from certain IP numbers (defined in block lists) to gain access to your computer. The whole purpose is that the blocked IP numbers usually belongs to this or that organization that wishes to find out more about your Internet habits and other information they have no reason to get their noses into.
Moblock has a big brother called Peerguardian by Phoenixlabs but development on this program seems to have been discontinued, and unfortunately at a stage where the program doesn’t work (at least for me it doesn’t). I am also sure there are some Windows variants of an IP-blocker (I’m guessing Bluetack is the right place to go).
Installing Moblock on Ubuntu turns out to be a very simple affair. Mainly do two things: Add moblock’s repository to your repository list, and run an apt-get command. (Here’s an even better instruction for installing Moblock on Ubuntu).
The installation takes care of setting up cron-jobs to update your block lists every day, installs moblock as a service started every time the machine is started, and makes the first download, after which the program (or in fact, demon) is started and you are safe.
The above link is a very good instruction on installing moblock, and it even have instructions on how to perform some simple troubleshooting.
If you run linux I suggest you take a look at moblock’s home page, or you can check out it’s project page on sourceforge.