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