A crash group lets you organize a set of call stacks using some level of the call stack hierarchy other than the top-most stack frame. Imagine you have the following three call stacks. By default, these would all be grouped using the top-most stack frame (stack key A), and you would have just one entry in the Summary page that grouped all three crashes.
If you set up a crash group at level 2, stack key A would no longer appear in the summary table. Instead, you would have three different groups displayed (B, D, and F).
To create a crash group, select a row of your call stack either on the Crash page or in our Stack Explorer, available from the Key Crash page. A group can be removed in a similar manner by selecting the top-most stack frame.
You can view the list of all your currently defined groups on the Groups page.
You can learn more about how crash grouping works here.