When you get a letter from the IRS, it's important to read it carefully and understand what they're asking of you.
If they're asking you to send them documentation to support specific items on your return, then you'll follow the instructions in the letter and send them what they ask for. In your case, they might want copies of your W-2s to support the amount of withholding you've claimed, or certain receipts for any credits claimed, or documentation to support business income & expenses. But you'll only send them documentation if they ask for it.
If they're simply advising you that they're reviewing areas of your return -- but at this time you don't need to send them any additional information -- then you'll wait until they ask you for something. Your return may be reviewed and the case closed with no further action required of you. Or they may review it and ask for additional documentation. In the meantime, you'll wait until you hear from them.