If you want to remove some of the characters of the filename, this is a good method.

Remove
Specify how many characters that should be removed. If non-numeric the value is used as a search pattern.

Starting at
Specify the starting point of the removal. If non-numeric the value is used as a search pattern.

Backwards
When checked, the Start position will be calculated from the end of the filename.

Use regular expressions
If checked the "Remove count" and "Starting at" text fields can be regular expression. The expresions are used as search patterns and translated to a numeric value.

Apply To
Specifies if the change should be applied to the name, extension or both.

Note: If you want to remove certain characters or words (like all "a"s), use the replace method instead.