explain query

It appears that columnName is a date in the tablename table which is being converted to a varchar.

Three expressions are then being returned by the query.

The date is first expressed in the 103 format:   dd/mm/yyyy.

4 characters from the 7th character onwards are then extracted i.e. the year.

The date is next expressed in the 100 format:  mon dd yyyy hh:miAM (or PM).

The first 6 characters are then extracted i.e. the month and day.

Finally, the date is again expressed in the 100 format: mon dd yyyy hh:miAM (or PM).
2 characters from the 5th character onwards are then extracted i.e. the day.