You have three years from the due date of the tax return to file for a refund. So you must file a 2015 return no later than April 15, 2019, to claim the refund. You can find prior year tax forms from the IRS at https://www.irs.gov/forms-pubs/prior-year.
A portion of earnings withheld by an employer or put into a retirement plan for distribution to the employee at a later date. If certain legal requirements are met, the deferred amount is not taxable until actually paid, for example, after retirement.