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.
A->B->C
A->D->E
A->F->G
If you set up a rule to ignore stack key A, crashes would instead be grouped at the second level of the call stack. Instead of a single group at A, you would have three different groups displayed (B, D, and F).
Crash report grouping rules can be changed on the Groupings page or by expanding a stack frame on the Crash page and selecting Group Rules.
For more information on how to group crash reports, please see Grouping Crashes.