"Arduino USB Terminal" is an arduino microcontroller shell that will allow you to give instructions easily from your smartphone to your Arduino using the USB and / or bluetooth protocol.
In order to use this app you need:
1) Android smartphone that supports OTG protocol
2) OTG cable
3) Arduino

Below is a very simple sketch example to send characters to Arduino:

char ReadChar;

void setup() {
pinMode(13,OUTPUT);
Serial.begin(9600);

}

void loop() {
if(Serial.available()){
carattereLetto=Serial.read();
if(carattereLetto=='a'){
digitalWrite(13,HIGH);
}
else if(carattereLetto=='b'){
digitalWrite(13,HIGH);
delay(3000);
digitalWrite(13,LOW);
}
else{
digitalWrite(13,HIGH);
delay(1000);
digitalWrite(13,LOW);

}
}



}

Below is a very simple example sketch to receive characters from Arduino:

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

}

void loop() {
Serial.println('a');
delay(1000);

}