What does 'Previous Period' mean?
The 'previous period' is the same length of time immediately before, e.g. if you're looking at 30 days, it compares it to the previous 30 days.
Note: As only 90 days of data is stored by default, if you set your report for a period of 3 months or over, there won't be data for a previous period to compare the current period with, which means the increased percentage for any modules will be 0%.