Example for roll up operation

I am finding hard time understanding the roll up operation. the example in the course was not clear to me…
if i find the example and the explanation for roll up operation then i can understand the concept…
anyone help me out please.

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I am just another mongoDB user wondering the same question.
With a very limited time searching for this answer, this is what I have came up.
It seems it is still under development:

If you can provide or share references to where you encountered your ROLLUP example, that might be helpful to put our notes together on this thread.