The falling-notes view is a timeline running down the screen. Each bar is one note: where it sits across the screen is the key, how tall it is is how long you hold it, and the moment its lower edge reaches the keyboard is the moment to play. Once that is clear, you can follow which key to press and when, even if reading notation is still slow going.
Position, length and brightness
An 88-key keyboard runs along the bottom, and every note falls straight down onto its own key. Press when the lower edge touches the keyboard; let go when the top edge has passed. A tall bar is a long note and a short one a short note - even the briefest note is drawn tall enough to see. The key itself lights up for as long as the note sounds.
Brightness shows how loud a note is: louder notes are drawn lighter, softer ones in a deeper shade, so you can see where a melody peaks and how far the accompaniment stays underneath it. Notes on black keys use a darker version of the same colour, to tell them apart from the white keys.
Settings (the gear button, or the E key) has three kinds of guide line under Display Lines. Octave Lines (C) mark every C and Scale Lines (E-F) mark the boundary between E and F - the two places where white keys meet with no black key between them, so you can name a note by counting from the nearest line. Beat Lines are horizontal lines that fall with the music, one per beat, with the line at each bar drawn heavier, so you can count as you play.
Colours follow the tracks in the file
Each track in the MIDI file gets its own colour. The app does not guess which hand plays what; it simply colours the first track blue, the second green, the third purple and so on. Nearly every piece on this site has one track for the upper staff and one for the lower, so in practice the two hands arrive in different colours. Some files have more tracks, such as an empty one with no notes, and then one hand can arrive in the third colour, purple.
Which staff comes first, though, differs from file to file. When you open a piece, watch for a few seconds to see which colour lands on the high side of the keyboard before you start. And where a note written on the upper staff is taken by the left hand, it keeps the upper staff's colour: the final say on which hand plays what belongs to the score and to you.
The colours earn their keep in hands-separate practice. Slow the playback down, follow one colour only and play just that hand along on your own piano. The app plays the other hand, so even practising one hand you hear the whole piece around it. If the notes crowd together, Note Speed in Settings changes only how fast they appear to fall, not the tempo.
The pedal display
The "Soft" and "Sustain" gauges at the top left show the soft pedal and the damper (right) pedal as recorded in the file. The gauges fill with the depth of the pedal, but the sound only switches on or off once the pedal passes a set point; it does not follow the depth. While the damper pedal is down, notes keep ringing after their keys are released, and while the soft pedal is down, everything plays a little quieter. Turn them on or off under Display > Pedal Indicators in Settings.
Bear in mind that many files carry no pedal data at all; the pieces on this site only have it where the engraving shows pedal marks. A gauge that never moves does not mean "no pedal" - it means the file says nothing either way, and the pedalling is up to the score and your ears. Chopin's Prelude Op. 28 No. 20, a C minor piece of chords in 4/4, is one that does carry it.
Switching to the sheet music
Pieces with sheet music can swap the falling notes for the score, with the button at the top right of the player ("Show sheet music") or Display > Sheet music in Settings. The notes being heard light up, a band marks the bar being played and the page turns itself; click a note or a bar to jump there. To return to the usual view, press the same button again (now "Show falling notes").
A good order is to learn the pitches and the shape of the rhythm from the falling notes first, then switch to the score and see how those same notes are written. That way a piece you have learned by ear and eye gradually becomes a way into reading notation. Practice mode always uses the falling notes, and a MIDI file you open yourself has no sheet music view.