What does Apple mean by sending it by SMS?

The other party's mobile phone is not an Apple mobile phone. The blue one in Apple's mobile phone is iMessage between iPhones, and the green one is ordinary SMS. There are many blue messages in the mobile phone, indicating that the other party's mobile phone is an Apple mobile phone and can receive blue iMessage. It only shows that a short message was sent, indicating that the other party's mobile phone is not an Apple mobile phone and can't receive iMessage, so it was sent by ordinary short message, which succeeded.