Hi pass passes anything above the filter's rated frequency, low pass passes anything below the filter's rated frequency. The attenuation curve or slope will begin near the rated frequency and a signal exactly at the rated frequency must only be attenuated by 3 dB or less. I dont know how to accomplish this design-wise. Google has many on-line examples of different filter circuit designs. I'm sure it has to do with capacitive and or inductive reactance relationships in the circuit.