To 'transpose' a piece of music is to change its key. That means that all the chords and melody lines and harmony shift, together, to new pitches. The main reason musicians transpose a piece of music is to accommodate the singer's range. Some people say that each key has a different 'color', or vibe, about it, but personally, I've never perceived any difference.

But singers are stuck with their vocal range, and if there are notes in the song that they can't reach, either high or low, the tune needs to be transposed to a key that suits the singer. To do that, we refer to a transposition table like the one above, until we can just do it mentally. That takes a while.