Calculus (or tartar) is plaque that has been left on teeth for a long time and gone hard. It builds up on teeth like scale inside a kettle, and this can make the teeth feel rough and look unsightly. It tends to form where the teeth meet the gums, and in between teeth, and you can watch this happening on the model. Also, you often see it behind the bottom front teeth (near where the salivary glands squirt out their saliva).
You won't always be able to tell if you have calculus, so you really need to go to the dentist to get your teeth checked regularly. Calculus is too hard to remove yourself with a toothbrush, and so it needs to be removed properly by a dentist or hygienist using special cleaning tools. You can help prevent calculus forming by keeping your teeth clean. Why not go to the oral hygiene part of 3D mouth to see how to do it properly.