You select the (Row Number) option in a column selector, when you want each row in the data table to be shown individually in a visualization.
Assume you want to show the age of each person in the data table above as bars in a bar chart. If you select 'Name' on the category axis and do not notice there are two Johns, the ages of these two persons will be aggregated as shown below.
Selecting (Row Number) instead of 'Name' on the category axis will present each row in the data table individually. The value 'John' is listed in row 1 as well as in row 4 in the data table, but the value is now represented as two individual bars, '1' and '4'.