When you open the MidiPiano, you will see a "virtual keyboard" under which is the "Select file" button, click it to select the midi file. If you do not have such files on your computer, you can Download Midi Files
on the Internet for free. Then press the keyboard keys to play MIDI notes.
This is not a complete app for beginners looking for an easy way to play midi piano for free. This software is still in development. Please do not hesitate to contact us to ask questions, report bugs and offer new features for playing on the Free Virtual MIDI Piano Keyboard Online.
What is a MIDI file, you can read here