It just means the IRS is in the process of reviewing your tax return. They are working on it.
Here is the general timeline of e-filed returns:
Transmission > Acceptance > Processing > Approval > Refund.
Please see the links below for more information:
Sadly the IRS tool only has 3 steps ... Received >> Approved >> Refund sent. A lot happens between the received and approved steps and it would be nice if there was more steps on the meter but there is not. The generic message that the return is being processed is all that is posted. How long the IRS will take to process your return is unknown. Although 9 out of 10 returns are expected to process within 21 days not all will be... check the tool every couple of days to get a progress report.