Stamina, also sometimes called movement, is a measure in general of how tired a character is.
Moving from room to room will decrease your stamina, traveling on the overworld map will decrease your stamina much quicker. Fighting also requires the use of stamina, with higher level forms requiring more stamina for each form use (for example, a level 7 form may require 7 stamina to work, whereas a level 2 might only require 2). Note that weight of weapon can also play a role in this.
1000 is the maximum amount of stamina that is possible, it cannot be increased. The endurance skill can slow the use of stamina in certain endeavors however.
Also note that traveling through the overworld while mounted will require significantly less stamina.
Stamina can be regained in 4 ways:
When a character's stamina reaches 0, that character will be knocked unconscious by the next thing which hits them. They are subdued at this point, and can be bound and blindfolded.
If a character attempts to fight and their stamina dips below 0, then they will quickly lose hitpoints until their stamina crests 0 again.