What is the betwwen [i,j] and [u,v] in explinations of correlation and convolution (picture attached)? I don't know how to write 'summation' symbol here hence posting the picture.

Can someone explain to me the difference between i,j and u,v in this explanation of correlation and convolution? 

I know this discussion must have taken place somewhere but it is difficult to look for such discussion by keyword 'correlation' and convolution'.

I have already referred to the following posts:
1. Difference between correlation and convolution on an image?
2. What is the difference between convolution and cross-correlation?

Adding the source for reference: https://www.youtube.com/watch?v=C3EEy8adxvc
Thank you in advance.
$[i,j]$ are your spatial coordinates and $[u,v]$ are summation indices. 

The difference between correlation and convolution is simply the sign: correlation has $i+u$ and convolution has $i-u$. 

They are close cousins: it's easy to express one as a function of the other. For example: correlation is the same as convolution with a properly mirrored/flipped version of the kernel. If the kernel is sufficiently symmetric, both correlation and convolution will yield the same result.