Groups are a concept best explained with a visual example. Let's say you want to view your Gantt chart in a way that shows all the tasks on the critical path together, and then within that group , shows tasks grouped by their Percent Work Complete values. Critical tasks can often be spread all over a view if it is shown in the normal ungrouped format of the Gantt chart.
This is how such a Gantt chart would look with a grouping as described here.
The Critical Tasks grouping has a yellow band , and those tasks with a Yes value (the tasks that are critical) appear at the top.
Within this grouping is a second set of groups that sorts the tasks according to their Percent Work Complete values.
Also note that the values of the fields shown roll up the group structure much like they would a summary task structure. This field shows us that for the entire group of critical tasks, the total Percent Work Complete value for the group is 2%.
The total Work for the noncritical tasks is 14 hours.
The Critical Tasks grouping has a yellow band , and those tasks with a Yes value (the tasks that are critical) appear at the top. 
Within this grouping is a second set of groups that sorts the tasks according to their Percent Work Complete values. 
Also note that the values of the fields shown roll up the group structure much like they would a summary task structure. This field shows us that for the entire group of critical tasks, the total Percent Work Complete value for the group is 2%. 
The total Work for the noncritical tasks is 14 hours. 