What is the meaning of the percentage used to describe the thinness of this glider's airfoil?
14.5% given in the article is the (maximum) thickness to chord ratio, expressed in percentage. It is usually used to explain how thin (or thick) the airfoil is. Note that this is a characteristic of the airfoil.
From the NASA page Wing geometry definitions:

Airfoil thickness to chord ratio, image from NASA page Wing geometry definitions

... The straight line drawn from the leading to trailing edges of the airfoil is called the chord line. ... The maximum distance between the two lines is called the camber, which is a measure of the curvature of the airfoil (high camber means high curvature).
The maximum distance between the upper and lower surfaces is called the thickness. Often you will see these values divided by the chord length to produce a non-dimensional or &quot;percent&quot; type of number.