Exactly what is means ... your return is being looked at and all you can do is wait for the process to be finished. If the IRS sends you a letter respond to it promptly.
The IRS no longer processes returns in first in
first out batches. They are processed individually so returns sent in at the
same time will not necessarily process together. The IRS states that 9 of 10
returns will process in 21 days or less. However considering that the IRS
processes more than 140 million returns that means 14 million will be delayed
for more than 21 days.
Think of the IRS process as a long pipe line. Some returns will be rejected before being allowed into the pipe and the rest will be accepted. Then once accepted into the pipeline many will process directly through without any delays and some will not. Some returns will be siphoned off for further review of some kind. The basis of what triggers these reviews is a well guarded secret but some are just picked randomly every year.
Some reviews are automated and some will require human attention. The processing times will vary but, due to the continued under staffing situation they are likely to be much longer this year.