You'll see a staff with notes sliding from right to left. Wait until each note "hops" into place on the left, then identify that note by clicking the correct button at the bottom.
If you get it right, the note will disappear and you can get ready for the next note. If you were wrong the drill will "shake", and you can try again if you're quick.
Watch out — if you don't get the note in time, it will get knocked off by the next note!