What does processing mean and how long does that take

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

Also see:

When to Expect Your Refund if You Claimed the Earned Income Tax Credit or Additional Child Tax Credit | Internal Revenue Service (irs.gov)