What does "Traffic in on switchports" show?

Hi I use NAV to, among other things, produce a monthly report about the usage statistics in our network. For this I use Statistics -> "Traffic in on switchports" + "Last Month". What does the resulting view show? Is it the average amount of traffic over the last month, or is it the total, or something else? I can't quite make the numbers add up on the different switch ports. Lars Kulseng