It is the highest value that a metric has reached since you began measuring it.
The term is a reference to the marks that periodic floods make on buildings (or landscapes).
In the Progress world the two most common high water marks are the amount of space used in a storage area and the number of locks used in the lock table. The current value is often less than both the maximum value and the largest value that has been seen. It is useful to know all 3 (current, maximum and "high water") in order to judge whether or not you need to do something like increase (or decrease) -L or add an extent to a storage area.