The Product function (in Excel) simply multiplies a set of numeric values.
In the finance industry at least, this function is commonly used to calculate total growth (or return) for a series, which can then be averaged, annualized or compared to other data series.
For example: if an investment returned 12% in the first year -6% the second and 9% in the third, the total return can be calculated by adding one to each return number and taking the product:
1.12 * 0.94 * 1.09 = 1.148 -1 = 14.8%
Simple but powerful and difficult to replicate in Tableau at present if you want to be able to change the calculated set based on filters.