Certificate request file created by Keychain Access, an Apple tool that manages and stores passwords and certificates; contains data referenced by Apple to request a signing certificate for iOS and Mac development.

You will need to upload your CERTSIGNINGREQUEST file to "https://developer.apple.com" (you'll need an Apple ID to login). This is necessary in order to request a Development and Distribution Certificate that enables you to generate .IPA files, which are needed for final distribution and uploading of your app(s) to Apple. To create the CERTSIGNINGREQUEST file: Select Keychain Access → Certificate Assistant → Request a Certificate From a Certificate Authority.... Enter the appropriate information in the fields, click Continue. Name your file (or leave it as the default "CertificateSigningRequest" name), choose the save location, and click Save then Done.