Look at C, then the black key just to its right. That black key is C sharp, written C♯. It is a little higher than C. The same black key can also be called D flat, written D♭, because it is a little lower than D.
Sharp means one key higher. Flat means one key lower. On this piano the black keys are the sharps and flats. Glow Keys prints C♯ and F♯ with a sharp, and E♭, A♭ and B♭ with a flat, so each key has one name.
A key signature is a little family of sharps or flats printed at the start of a piece. They stay on for the whole tune, so the writer does not have to mark every single note. C major, the key used for Twinkle’s chords, has an empty key signature. That is why the tune fits on the white keys.