Using the Insert and Delete you can insert a character or string at any character position that you select, or you may Delete Number of Character From particular character position.

So,  for example, if a group of files is named 1, 2, 3, 4, etc., you can insert " pic " at character position 1 to rename them: pic1, pic2, pic3, pic4 etc.,

If a group of files is named thumb1, thumb2, thumb3, etc., you can shorten the names to th1, th2, th3, etc. by deleting from character position 3, for a character count of 3 (Number of Character).

By default application insert and delete character from start of the file name. You can start from backward if you check checkbox "Insert from Backward" and "Delete from Backward"