Serial.available()

Get the number of bytes (characters) available for reading from the serial port. This is data that's already arrived and stored in the serial receive buffer (which holds 64 bytes).
Serial.available();
the number of bytes available to read
int incomingByte = 0;   // for incoming serial data

void setup() {
    Serial.begin();     // opens serial port, sets data rate to 115200 bps
}

void loop() {
    // send data only when you receive data:
    if (Serial.available() > 0) {
        // read the incoming byte:
        incomingByte = Serial.read();

        // say what you got:
        Serial.print("I received: ");
        Serial.println(incomingByte, DEC);
    }
}