It divides the contents of L13 by contents of F13 and raises the result to
the power of 0.166667 [which equates to finding the 6th root of the division]
and then subtracts 1.
I would expect the formula is calculating Compound Average Growth Rate. If so,
L13 would be the earnings, F13 is the amount invested, and 0.1666667 is the term
(ie, 1/6 of a year or two months).