Very Basic Arduino Uno Serial.readString() operation
If you set your Serial Monitor's line ending to "Both NL &amp; CR", then this code will find the word "send". It will NOT find the word "send" in this string, "ssend" or in this string, "sendd".

void setup(){
  Serial.begin(9600);
}

void loop(){
  while(Serial.available() &gt; 0 ){
    String str = Serial.readString();
    if(str.substring(0) == "send\r\n"){
      Serial.println("identified");} 
    else{
      Serial.println("unknown");
    }
  }
}


If you want to find the string "send" in "1234send" or "send1234" or "1234send5678" then use indexOf();

void setup(){
  Serial.begin(9600);
}

void loop(){
  while(Serial.available() &gt; 0 ){
    String str = Serial.readString();
    if(str.indexOf("send") &gt; -1){
      Serial.println("identified");} 
    else{
      Serial.println("unknown");
    }
  }
}