Officially the Hash is a drinking club with a running problem. There are thousands of different hash chapters over the whole world. To cut a long story short, a hash is a run. Each run is around 10 kilometres long and all you need to do is show up at the run location at the right time and you're in. It is not about being the fastest, it is more about having a good time while exercising. For more detailed info you may want to check out the "what is hashing" chapter.