To add on to what's in hawk900's tutorial on the "Art - Animation - Run Speed and Art - Animation -Walk Speed", the Run Speed will also basically state at what movement speed a unit will activate their Walk (Fast) animation, and anything lower will cause them to move using just the standard Walk animation.
Most units only have one movement animation, but for some (Such as Kodos, Thunder Lizards, Magnataurs, and Mammoths) there's an animation for both.
As an example, let's say a unit has a Run and Walk speed of 200. Whenever their movement is above 200, they'll use the "Walk Fast" animation. Whenever their movement is reduced below 200 (usually by a slow effect) they'll resort to using their "Walk" animation.
At least that should be how it works. I just messed around with it now in worldedit and it seemed to work like that. Just mess around with those values some until you get your desired effects.