They are probably referring to a tax transcript for the IRS (they may not have a copy of their prior-year tax return(s), so they request needed information from the IRS.)
To obtain a full copy of your filed and processed tax return (including income forms such as W-2s and 1099s), print and complete IRS Form 4506, Request for Copy of Tax Return. Be sure to follow the instructions on page 2 of the printout. The IRS charges $50 for each copy. Please allow up to 75 days for delivery.
If you can't wait that long or don't want to pay the $50 fee, consider a free IRS transcript. A transcript contains most line items from your return and can be used if you need last year's AGI or other numbers from a previously-filed return. Here's what a transcript looks like. To order, go to the IRS Get Transcript page and choose an option: online (same-day) or by mail (5-10 days).