could you explain this formula please...

Guys this formula is about dot product ...

1. I cannot understand why 1/n? Usually I haven't seen this part in other dot product formulas
2. If we assume, we have two vectors a=[1,2,3] and b=[3,2,1] -> n is the total number of elements, which 3 in this care? and j is the current element e.g. j=1, implies a=1 and b=3

Thank you guys...