Of you connect an Arduino to a PC, it presents itself as a serial port (on Linux it will show as e.g. /dev/ttyS0 or something in the line, I have not used Arduino in combination with Linux).
So now you send data from the Arduino to the PC. Your PC has no idea what to do with it; you need an application that handles what the Arduino sends. That can e.g. be minicom (a Linux terminal program) that displays received data on the screen or an application that understands e.g. ‘words’ like ‘next’, ‘pause’ etc. that are received on the serial port by that application and knows what to do with them.
If you use an Arduino that can act as a HID, you can control an application that runs in the foreground (e.g. VLC) by sending keystrokes like O to open a file or other shortcuts to play and pause. Or you can send text to e.g. gedit.