Generating a truly random number is surprisingly difficult. Creating enough apparent randomness for something like video game behavior, or other generally inconsequential behavior isn’t too hard, but for encryption and other important applications, this can be a challenge. Z1FFER addresses this as a true random number generator (TRNG) shield for the Arduino. It uses a technique known as Modular Entropy Multiplication to generate these numbers.
According to the product description, the motivation for this device came from the fact that (according to the Snowden documents) the NSA works to insert vulnerabilities into commercial encryption systems. This could include a random number generator, so the Z1FFER aims to combat this as an open source generator that works using thermal noise.
Besides the product page linked above, you can also check out the OpenRandom.org website to see how to build one and get more information on the technique they are using. As stated there, even the output of this board isn’t entirely random, but no random number generator is. Likely it is very good, however, as an open source product, you’re free to examine and modify it for yourself!