What does it mean when you recieve a letter from the IRS saying they are conducting a thorough review of my return and what can I do to speed up the process
It simply means they are processing your return.
This has been common this year.
The IRS started processing returns later than in prior years and as such has a backlog to work through. They also just recently started processing returns claiming EIC and/or ACTC.
During processing, the government reviews your refund. 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. At that point the IRS site will update with a direct deposit date.
Unless the IRS requests specific information from you, there is nothing to do on your part.