It is an IRS Form to request a copy of a tax return.
Here is the IRS link to the Form 4506: IRS Form 4506
If you did not keep a copy of your return, when you filed, and now need the return itself, this is how you get it.
Many need to only know some numbers from the return and will use a IRS Form 4506-T online at irs.gov to order a IRS transcript.
