Musical notes are sounds that travel as sound waves. How frequently one of the waves travels to your ear per second determines how high or low a sound is. The higher the frequency, the higher it sounds.
So, what is the frequency of each musical note?
A musical note that is one octave higher has twice the frequencies.
For example, the middle C note (C4) has a frequency of 261.6, and C3 is one octave lower than C4. C3 has a frequency of 130.81 which is half the frequency of C4. C4 is a higher pitch, and has twice the frequencies of C3.
C3 = C in the third octave
C4 = C in the fourth octave (also called the middle C)