It simply means that your e-filed return is being processed and that the government hasn't approved or rejected it yet. Sometimes return status does not update for several days or even longer.
During this time, the government reviews your refund (processing).
During the review process, they look for math errors on your return (extremely rare in TurboTax) and they also check if you owe back taxes, unpaid child support, or other debts.
If they need to make any corrections, they may
offset (reduce) your refund. In some cases, the correction might even increase your refund.
Once that part's done, the government approves your refund, which means it's ready to be deposited or sent.
It can take up to 21 calendar days after acceptance for the IRS to issue your refund, although most refunds go more quickly than that, while a small handful may take a bit longer.