A group is collection of individual items under one heading.
There are groups in the filters and you can create groups for accounts.
If you look at the setting/filter options when you are creating/editing a report or pulling up a list, you will see group names and then the individual items underneath the group. An example would be under the Categories filter. You have groups like Automotive, Beverages, Services and the like.
When you look at the group name there is a little black triangle in front of it. If you click on the triangle, it will expand the group list and you would then see the individual items that are contained in that group. These items are signified with a circle/dot icon in front of each one.
You can select the entire contents of the group by checking the group name or you can select specific items within the group by unchecking the group name and checking the checkbox for the individual items that you want to see