kHz or ‘kilohertz’ is a measurement of the frequency of the sonar signal (number of cycles per second). Like you measure the size of a memory card in ‘bytes’, where 1,000 bytes is written as 1KB. Sound pressure is measured in ‘hertz’ (Hz), so kHz is 1,000 hertz (or 1,000 cycles per second). The higher or more hertz there are, the higher the frequency. The lower the hertz, the lower the frequency. Generally speaking; the higher the frequency the more detailed the sonar image, but the lower the range (distance) it will work to.

Not sure what you are meaning by this: “So the number such as 75 kHz would be over 30 feet”.