iOS Distribution Certificate is required for App building and App launch operations.
Situation 1: If your App is already launch on the App Store:
1) If your App does not have an update plan, you can ignore this reminder. The expiration of this certificate will not affect the use of your online App;
2) If your App needs an updated version recently, you can refer to the tutorial Create Certificates for iOS, regenerate the certificate, and rebuild; Situation 2: Your App didn't launch on App store yet:
Since this certificate is required for both iOS construction and launch, the expiration of the certificate will cause App build failure. You can refer to the tutorial Certificates for iOS. After the certificate is generated, you can rebuild your App again.