"Lingual Surface" refers to the surface of the tooth that is next to your tongue
Overall, there are five surfaces to each tooth:
Occlusal - the chewing or grinding surface of the bicuspid and molar teeth
Mesial - forward or front.
Distal - behind towards the back of the mouth.
Buccal - the tooth surface which is next to your cheeks.
Lingual - the aforementioned surface next to the tongue