The Pitch filter lets you change how high or low the sound is with respect to frequency (not loudness).
One may use this effect to disguise a voice or create a silly sounding voice. It is very useful to use in conjunction with the Speed option in Properties, which inherently changes the pitch. Thus, this filter can be used to correct the pitch. You can simply copy the speed value from Properties to the Speed Compensation parameter in the Pitch filter!
The Octave Shift parameter is a different way to represent the change in musical form. There are eight whole steps in music between each octave. Thus, each whole step corresponds to increments or decrements of 0.125.