Ssrs 2010 group by statement

In an existing ssrs 2010 report thtat I need to change, there is the following expression in the group by =CEILING(RowNumber("Group3")/1)
by properties of the Row group called 'Details3'. The group3 in the expression that I just listed, references all the detail rows that appear on the report.

Thus can you tell me what the '=CEILING(RowNumber("Group3")/1)' expression is actually doing?