Once the tax return has been initially e-filed it can either be Accepted or Rejected. If rejected the user can correct the error and e-file again. Once Accepted the tax return is processed by the IRS. If the IRS finds a problem with the tax return during Processing they send a letter or notice in the mail to the taxpayer describing the problem and what they need the taxpayer to do.
After the tax return has been Accepted by the IRS (meaning only that they received the return) it will be in the Processing mode until the tax refund has been Approved and then an Issue Date will be available on the IRS website.
See this IRS website for federal tax refund FAQ's - https://www.irs.gov/refunds/tax-season-refund-frequently-asked-questions
