Pitch distance means: how far one pitch is above (or below) another
pitch. We measure pitch distance in half-steps. A half-step is the
distance between two pitches that are right next to each other, with no other
pitch between them, including the sharps/flats ("black" pitches).
Important: Two pitches that are right next to each other on the piano
often do not make a half-step. For example, the white piano keys
A and B are "next to each other", but they don't make a
half-step, because the black key A♯/B♭ is between them
Here are some example half-steps:
A to B♭
B to C: Neighboring white keys do make a half-step if there's
no black key between them.
C♯ to D
(By the way, "half-step" is the American term; in the U.K., a semitone
is the same thing.)
A whole-step is a pitch distance equal to two half-steps. For
Consider the 3 pitches C-C♯-D.
C to C♯ is a half-step; C♯ to D is
also a half-step.