Pitch is the 'high' and 'low', or the 'up' and 'down' of musical notes. Pitches going up or down step by step to make a scale. Put scientifically, pitch is just the frequency of the sound vibration - given in hertz or cycles per second.
Pitches turn into music when they are arranged in some way.
Different pitches one after another can make a melody
Different pitches combined together can make harmony.