It looks more daunting than it really is. If my song is written in the key of D but I find that it doesn't suit my voice, I experiment with a capo and find it works best at the 3rd fret. So according to this chart, I read it as a graph and find that D3 means the song is now in the concert pitch of F. If I am playing with other instruments, they would need to know the new key when a guitarist is using a capo.
Of course, there are no such keys as D#, F#, G#, or A# so just go by the enharmonic equivalents for those (Eb, Gb, Ab, and Bb).
