The top section of the Rename dialog is where you define the rename mode - which determines the way the two input fields, old name and new name are interpreted.
Use the Mode drop-down to select the rename mode.
Other controls in this section are:
The Reset button provides a quick way to clear out the other fields of the Rename dialog and revert to a "blank slate".
More:
Standard Wildcard Rename
Find and Replace
Regular Expressions
Regular Expressions + Find and Replace