If you received these Letters of Communications from the IRS, it clearly states what each of the Notices represent.
CP-515 and CP-518 are a part of the IRS's CP series of forms sent to taxpayers to inform you of different actions and request.
The CP 515 deals directly with "request for your tax return" and CP-518 is sent when taxpayers don't respond to CP-515 and explains that you have committed a crime by not filing your past due tax return.
If what you really want to know, is how serious these notices are? They are VERY serious and you can contact the IRS at 1 800 829-4477 or contact a tax professional as soon as possible.