A strong set is a set of keys in which every key can reach every other key in the set by some path. There are many strong sets in the Web of Trust, and the largest of them is called the strong set.
The reachable set is the set of keys that have a path to them from the strong set. That is, every key in the strong set can reach every key in the reachable set.
MSD stands for Mean Shortest Distance. It's the average distance from every key in the strong set to this key. The lower the key's MSD, the more reachable this key is.