Reference → Serial →

Serial.read()

Reads incoming serial data.

Serial.read();

the first byte of incoming serial data available (or -1 if no data is available) - int

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);
    }
}