At this point, it is impossible to know exactly what will be in the letter - that will depend on what errors the IRS is reviewing and their findings.
Yes, if the IRS sends you a letter, they will tell you what action is required on your part. Often, they will request additional information. They may also tell you their review did not result in any change to your return.
If the error is obvious (e.g. you forgot to report a W-2 or 1099), they will propose an increase in your taxes to cover the unreported income.
