Distributed computing is a way to share a task between several computers.
Normally big complex computer jobs are given to a big super-computer. Distributed computing is aimed at doing the same task, but by using a network of smaller computers.
RC5 is one such task. It is an attempt to decode an encrypted message encoded using the RC5 technique. There are other projects such as trying to find prime numbers, analysis of radioactive storage, AIDS research, etc.