The letter part of the code refers to different parts or surfaces of the tooth. An “M” mesial, or “D” distal, is the front or back surface of the tooth, respectively. An “O” occlusal, is the top or biting surface of a back tooth (molar or premolar), and “I” incisal, is the biting edge of front teeth (incisors and canines). A “B” buccal, is the surface of the tooth towards the cheek, and an “L” lingual, is the surface of the tooth towards the tongue. So, if the dentist says number 3MOD, you’ll know that you have a cavity on your upper right first molar, involving the front, top, and back parts of the tooth.
Source: http://www.dentistry.com/