it absolutely speaks volumes about your health! Ayurvedic medicine practices have talked about this for thousands of years. I'd suggest books by Doctors Vasant Lad or Robert Svoboda.
For example, if the tongue is coated with a white film, it indicates that ama (roughly translated as toxins) exists in the large intestine, small intestine, or stomach depending on which part of the tongue is coated. A pale tongue may indicate an anemic condition, and a yellowish tongue may indicate excess bile in the gallbladder or a liver disorder. Blue (and not after blueberries) can indicate a defect in the heart.
It also gets deeper into which parts of the tongue correspond to which organs, and what the different lines may mean. Really amazing stuff.