The ratio between the wingspan and the width of the wing / a sail or the wing of the wind/wing foil. A higher aspect ratio (relatively large wingspan) gives a better top speed and range, a larger AR makes a wing more maneuverable.
There is no 'hard' number where we call a wing/sail or wing of a foil 'High', 'Medium' or 'Low' aspect. As an indication:
Low aspect: AR of 5.0 or lower
Medium aspect: AR between 5.0 and 7.0
High aspect: AR from 7.0 and larger