The article is basically saying that by drawing the object, you look at it / think about it in more detail and it gives the mind more 'things' (hooks) to remember about the object.. and therefore, you are more likely to remember it.
Contextual representation means 'representing something in context'. 'Representation' in this phrase could refer to the act of drawing or the actual drawing (picture), both of which increase focus on the object, and hence improve memory (according to the article. 'Contextual' could refer again to the act of drawing the object, or where you drew it etc. or it could refer to the 'context' in which the object is drawn.. e.g. imagine if your brother just brought a new car, and you were asked to draw a car.. you might draw a picture of a car with your brother next to it.. which gives it context, and makes it more memorable!