Trends show you the '% change' up or down, on all of the data tables we have within the platform.
The comparisons will always be made against the same period you are measuring.
For example; if you have 'last 30 days' as the date ange set on the picker - you will be comparing the most recent 30 days, against the 30 days prior to that.
If you are selecting 'last quarter', you will be comparing against the quarter prior to that - etc.
If you would prefer to switch this feature off, you can do so by checking/unchecking the box labelled 'show trends' available on most of the screens with data tables.