Reference → Serial →

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). 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 (default receiving buffer holds 128 bytes). You can set you own buffer by means of begin() method

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);
    }
}