Sound is a longitudinal wave of vibrating particles, so it needs a medium to travel through. The frequency of a sound is the number of waves per second, measured in hertz (Hz). Humans hear sounds from about 20 Hz up to 20 000 Hz.
Sound with a frequency greater than 20 000 Hz is called ultrasound. It is above the upper limit of human hearing, so we cannot hear it, but it is still a sound wave and it carries energy through a medium in the same way as audible sound.
Wave speed = frequency × wavelength. In the same material, a higher frequency therefore means a shorter wavelength, so ultrasound has a shorter wavelength than audible sound. This is one reason it is useful for detecting small features.