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%.