Cannot understand program

[Blitzor DDD]

Hello!

I am trying to dissect a program and got stuck with one function:

void SerialPortHandler::sendTrackPause()
{
//    qDebug() << "sendTrackPause";
    QByteArray  byteArr;
    QDataStream out(&byteArr, QIODevice::WriteOnly);
    out << ActionType::Track << TrackType::Pause << '\n';
    serial->write(byteArr);
}

void SerialPortHandler::sendTrackPause()
{
//    qDebug() << "sendTrackPause";
    QByteArray  byteArr;
    QDataStream out(&byteArr, QIODevice::WriteOnly);
    out << ActionType::Track << TrackType::Pause << '\n';
    serial->write(byteArr);
}

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serial is an object of QSerialPort.
This function clearly pauses a player.

But what's going on under the hood?
For what sake do we need QByteArray and QDataStream?