Form 8453 is a form that you need to print out and mail to the IRS along with one or more other documents that could not be e-filed. The reason that you need this form, and what other forms you need to mail to the IRS, will be included in your instructions. Two common examples are form 8283 for non-cash donations over $5000 that require and appraisal, and form 8332 dependent release form when you are claiming a dependent who does not live with you. Other examples are printed on form 8453.
If you are required to mail any paper forms to the California FTB after e-filing, that will be on your California instructions.
