
Hashing is a non-competitive running event where runners chase a trail laid by a lead runner called a hare. You don't know the trail in advance, and it can go through some occasionally rough terrain. Trails are typically 5 to 10K in length and are of varying difficulty. At the end runners gather for a beer and a few laughs.
The name Hash House refers to the restaurant in the boarding house used by the founders and has nothing to do with any substances of ill repute.