Product Activity screen shows the numeric or financial results for the customers who have bought this particular product over the last six or twelve months. The number of months that are shown can be set in your settings screen. The values that are shown can be the sales value, the number of units or the margin that has been made.
The customers can be grouped into the different groupings of customers. Drop that down and you will have a choice of the customers grouped together who have bought this particular product. If you double click on a number, you will be able to see the transactions behind that number in a separate screen.