Low Potassium levels

Hi Everyone!
I just received a letter from the vet indicating that my dog's potassium level is low. it's 3.3 when it looks like it should be between 3.9 and 5.7. No where on this form does it say what a low (or high for that matter) potassium level means. It just states that he should be going on a ptassium supplement called Therea K which is a liquid added to his food once a day.
Is this really something I need to supplement? What does all this mean? Any advice is greatly appreciated. And if I should be supplementing, is their a natural supplement?
Thanks so much for all your advice and opinions :O)