"Not drawn to scale" means that the proportions of figures in a picture are not necessarily accurate. For instance, if they show you two lines where one is supposed to be twice as long as the other in real life, a scale drawing would show one line that is twice as long as the other, like 2cm and 4cm lines representing 4 ft and 8 ft lines. If the picture were not drawn to scale, then 4 ft and 8ft lines might be represented by 2 cm and 3 cm lines, where the longer is not twice as big as the shorter.
