In period mode, the first measure column after the Properties shows the total of the selected periods. This makes sense for streams that have cumulative data, such as 'sales invoices', but in streams that hold a snapshot of data showing the variance over time, such as 'stock on hand', the total is meaningless. For example, if I have ten of an item on Monday, Tuesday and Wednesday, sell two, and then only have eight on Thursday and Friday, by the end of the week, I don't have '46', I still only have eight.
On this type of stream, you can display the period total as an average, rather than a sum of the periods, via Options > Totals as average. So, in the situation described in the above paragraph, the first measure column would show the average as '9.2'. This does not change the first 'totals' row of the view, just the totals column.
The screen shots below show the effect of using Totals as average view. In the first image, the 'Total' column displays the cumulative value of three month's worth of sales. In the second image, the totals column displays the average of those sales.
Typical view
Totals as average view