Tap a, then tap the one above it. The second note sounds like the same note, just higher. That is not an accident. It is the one rule that pitch is built on.
A note is the air wiggling back and forth. How fast it wiggles is its frequency, counted in hertz: wiggles per second. The first a wiggles 220 times a second. The second one wiggles 440 times a second, exactly twice as fast. Look at the picture: twice as many waves fit in the same window. Double the speed and you get the same note again, one octave up.
what hertz means
One hertz is one wiggle per second. It is named after Heinrich Hertz, who studied waves in the 1880s. People can hear from about 20 hz, a deep rumble, up to about 20,000 hz, a thin whistle. Most of music sits between 30 hz and 5,000 hz.
the kick and the bass are low frequencies
Your kick's body hums at around 50 hz. A bass line lives between about 40 hz and 200 hz. That is why they are the sounds you feel in your chest: slow, big pushes of air.