A CP24 notice means that the IRS made changes to your return and you may have an overpayment credit. They found a difference between the amount of estimated tax payments on your tax return and the amount they posted to your account. This notice could also cover other changes the IRS made while processing your return.
https://ttlc.intuit.com/community/irs/help/i-got-a-cp24-notice-from-the-irs/00/27161

