Question

How do double sigmas operate on the variable? Can’t find it on google.

Answer 1

You start the index on the outside summation at the lower limit, then you  cycle through all the indices on the inside summation (the second one); then you increment the outside index and do another cycle.   If the outer summation is 1 to n  and the inner one is 1 to the current value of the outer index, for example, then you  would do an increasing number of iterations of the inside summation.