int relay=2;
String data;
void setup()
{
  Serial.begin(9600);
  Serial.print("welcome to my project:");
  pinMode(relay,OUTPUT);
}
void loop()
{
  if(Serial.available()>0)
  {
    data=Serial.readString();
    data.trim();
    if(data=="relay on")
    {
      Serial.println("relay is on condition");
      digitalWrite(relay,HIGH);
    }
    if(data=="relay off")
    {
      Serial.println("relay is off condition");
      digitalWrite(relay,LOW);
    }
  }
}