Semitones, keys, and what to pick
A semitone is one piano key. Common jobs: -1 to -3 brings a song down to a comfortable singing range; +1 fixes a track recorded a half-step flat; -2 matches a song in D to a guitar playing open-position C shapes with a capo off. Work out exact distances with the transpose calculator, check the result against the note-to-frequency table, and if you are shifting to sing along, find your range first with the vocal range test.