The STATISTICS toolbar button displays the Record Selection window, which allows you to display the number of members that meet the criteria you choose.
A similar window is displayed to allow record selection by criteria in other circumstances, such as via the Reports|User and Function|Listings|Members menu items from the Main Window.
Criteria for selection can either be treated as And conditions or Or conditions, depending on which item is checked. The Or condition works to combine those records meeting any of the conditions checked. For example, if you asked for a count of all members who are Associate Members Or Full Members you would get the sum of all Associate Members and Full Members. The And condition works to list those records that meet all the criteria checked. If you asked for a count of all members who are Associate Members And Full Members, of course you would get zero returned, since a member cannot be both an Associate Member and a Full Member at the same time. Thus the And conditions apply to the same member and the Or conditions apply for all members.
The Statistics window looks like:

As shown, you can choose matches based on: Tribe, Member Type, Sisterhood Type, Status (such as paid-up, board member, etc), Sign-up date, Birthdays, and marital status.