Diffraction is light bending around an obstacle. Roughly, the wavelength of light must be larger than the size of the obstacle for light to bend around the obstacle. So if the wavelength of light is just "infinitesimally" bigger than zero, it will not bend around any obstacle with a finite size.
I can't get the concept why there is no diffraction when lambda tends to 0 even for extremely small aperture.please explain simply.
A "ray" is a totally artificial concept which often works well when doing calculations to predict what waves will behave like.
To be strictly accurate - there is always some diffraction. Once you have decided how much diffraction is acceptable (e.g. how much energy appears in the sidelobes of a parabolic microwave reflector antenna or what resolution your optical telescope needs) you can determine the width of aperture you will need for a given wavelength.
The passage in that book is trying to put it in perspective. In the end, you are considering the ratio of wavelength to aperture. As the aperture gets less then so must the wavelength.
P.S. You presumably have read outside that particular passage in the book - concerning resolution and 'diffraction limit'.
