A capo clamps across all strings at one fret and acts like a relocated nut: every familiar open chord shape stays exactly the same, but sounds higher overall — one half step per fret.
What a capo actually does
A capo on the 2nd fret transposes every chord you play up two half steps, without changing the shape at all: a familiar open G major then actually sounds like A major. That’s the same transposition idea as the cadence degrees — just thought of mechanically on the instrument instead of in chord names.
Typical use: adjusting to a vocal range
A song often sits most comfortably in a particular chord shape (open chords instead of barre), but that key doesn’t match the singer’s range. The capo resolves this conflict: the comfortable shape stays the same, while the actual pitch is adjusted to fit the voice — just as handy for quickly matching another instrument or a recording that’s in an inconvenient key.