Activity Counter are the numbers shown in 3 circles at the top of the Activity Manager.
The above means that there are a total of 120 exportable pledges (Active + Pending - Gift) for the date(s) selected in the calendar. The average age of the 120 pledges is 42 years old. The average amount of the 120 pledges is $29.
The cog mark at the top right corner of the Activity Counter field lets you filter the contents to show in Activity Manager by campaign and channel. By filtering contents, you can view acquisition totals and stats for the selected filter (e.g. channel) for the selected dates (a date or a date range selected in the calendar).
TIP: Non-exportable pledges (Draft, Cancelled and Gift) are not considered for the counter.