Distal refers to the surface of the tooth that is towards the back of your mouth.
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 - The aforementioned back of mouth
Buccal - The tooth surface which is next to your cheeks.
Lingual - The tooth surface next to your tongue.