Transposition
Transposition is moving a tune to another key, higher or lower. The steps between the notes stay the same and only the starting note changes, so the tune sounds the same at a different pitch.
Take a tune in C major: G, F, E, D, C, B, C. Its steps are whole, half, whole, whole, half, half. Move it to G major. There the tune has to end on G, so it starts on D: D, C, B, A, G, F♯, G. The half step below G is F♯, the very black key from the G major key signature.
When you transpose, every note moves up (or down) by the same interval: here, all of them rose a fifth. In written music you see it in the key signature: C major has none, G major one sharp, F major one flat. In F major the tune would start on C: C, B♭, A, G, F, E, F.
Why transpose? To fit a song to a singer’s voice, or to make it easier to play. You can do it yourself: count the steps of the tune and repeat them from another note.
What it sounds like
A transposed tune is recognised at once: the same steps, only higher or lower – like the same song sung by a different voice.
Where it is taught: the studio’s interactive textbook, chapter 7, “G and F major: key signatures”.