In '5 stop VR reduction compensation' what does 5 stop means?

This is in relation to buying Nikon lenses and heard this term in AF-S NIKKOR
70-200mm f/4G ED VR lens review.

Already bought a 50mm 1.8D Prime.

Rule of thumb says that to avoid camera shake when handholding a lens you need to use a shutter speed faster than 1 divided by the focal length. (on a full frame camera)
So at 70mm - faster than 1/70th second and at 200mm faster than 1/200th.

The 5 stop VR allows you to handhold 5 stops slower. So taking the 1/200th
1 stop slower is 1/100th
2… 1/50th
3… 1/25th
4… 1/12th
5… 1/6th

That means that on a full frame camera you can hand hold that lens at 1/6th second with no visible camera shake.

Back in the day, the general rule for holding a camera + lens was that you should shoot at 1/focal length shutter speed to avoid camera shake (they didn't have image stabilisation (IS) or vibration reduction (VR) then).

So, if you were using a 200mm lens, the shutter speed should be 1/250 sec (nearest whole stop) as a minimum. If it was a 400mm lens, then shutter speed would be 1/500 sec (nearest whole stop).

Just so you know, whole stops of shutter speed are;
1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000

The idea of VR (IS) is that you can handhold the lens at slower shutter speeds than before as the VR counteracts any 'wobble' from camera shake. A 5 stop improvement would mean, in theory, that you could handhold a 200mm lens at 1/8 - not likely, you'd have to have a very steady hand & firm stance. In practice probably 1/60 sec is more obtainable.

The advantage of shooting at slower speeds is that it's better on overcast days - a high shutter speed would give you underexposure.