Can you please explain me what is 4.5 - 5.6G in a lense description.And how it is different from other generation of like 3.5G or 6.5.
"Nikon AF-S DX NIKKOR 55 - 300 mm f/4.5-5.6G ED VR Lens"
That simply refers to the Maximum Aperture of the lens.
The terms in that lens name are as follows:
So generally none of that refers to the Generation of the lens, just the features and specifications of the lens. There may be a "II" on the end of some lens names indicating that it is the 2nd version of that lens.
There are other acronyms you will see from time to time, there is a good reference on the Nikon site: https://support.nikonusa.com/app/answers/detail/a_id/9919/~/glossary-of-nikkor-lens-terms