This is a very simple wind instrument with a mouthpiece on one end and a slide on the other. Moving the slide out gives a lower pitch while moving it in gives a higher pitch. Note that moving the slide in decreases the distance the air may move, which effectively decreases its wavelength and increases the frequency of the sound, which in turn creates a higher pitch. The opposite, of course, is true if one moves the slide out, decreasing the frequency and pitch of the sound.
