If you run Magento store and offer to your customers USPS shipping options, you probably found out that it recently stopped working (it applies to Priority and Express mail shipping options). USPS was sending to its users an email about the change in their API, but if you didn’t take any action yet, it is time to do so.
Magento software developers created patches to address the issue. You can find all the necessary information in this post Magento Community Edition Alert: Important Patch for USPS API.
In short: you need to upload the appropriate for your version of Magento patch into your Magento root directory and run the appropriate SSH command, for example: sh PATCH_SUPEE-1868_CE_126.96.36.199_v1.sh.