Filtering by a property or properties in the Insights report allows you to select the criteria that you would like to be included in the report. Filters enable you to drilldown into a specific data set.
The groupBy function allows you to the view the grouping of your selected event(s) by a specific property.
Let’s say you have a gaming app and you want to see how many of your users have completed a level. If you select Uniques for a Level Completed event and then filter on Level 1, the Insights report will show you how many unique users completed Level 1 in your selected time frame.
Let's say you want to see how many of each level has been completed. You would select the Level Completed event from the dropdown and then groupBy Current Level, which will calculate how many of each level has been completed in the given time frame.