This is probably called a "letter audit". This is the most common type of communication from the IRS, where the IRS has noticed some apparent discrepancy on your account, and wants to ask you about it. This does not necessarily mean that you are being invited to some IRS office for an office audit (but read the letter).
Note that the IRS is happy to have you supply any evidence that supports your situation. This is true even if they ask you to pay some tax; you are encouraged to challenged the IRS' findings if you have other evidence.
Whenever you get a letter from the IRS, please visit our TurboTax Audit Support Center. They will explain the letter to you and suggest how to respond to it.