You enter your current mailing address on your tax return that is being filed with the IRS. The IRS needs the current address so they can contact you if ever necessary through the US mail. The address on your tax return and the address shown on any Form W-2's or Form 1099's do not have to match the address on your tax return.
Also see this IRS website for address changes - https://www.irs.gov/faqs/irs-procedures/address-changes
