The number of cycles per second (Hertz) (abbreviated as, hz) of anything that oscillates is called the "frequency". The electricity of an AC wall outlet is said to have a frequency of 60 Hertz as it cycles negative then positive 60 times each second.
Sound is an oscillating wave, but it has a broad range of frequencies. A low frequency sound (say, 50 hz) might sound like a low rumble, while a high frequency sound (say 12,000 hz), might sound more like a "sizzle". A person with normal hearing can hear all the way up to about 20,000 hz.
Sound is actually more like "compression" waves, rather than waves on the ocean. When something makes a sound, the air is compressed or rarified in waves that travel out from that source in all directions. When those compressed or rarified areas of air hit your eardrum, it vibrates in sympathy with those compression waves in the air and allows you to hear.
The higher the frequency, the shorter the distance between each successive compression (or rarification) in the incoming sound wave. This distance is called the "wavelength". Sound travels at about 750 miles/hour, so the compression waves between 100 hz and 20,000 hz have wavelengths that range between several feet (for the 100 hz sound) to a fraction of an inch (for the 20,000 hz sound).
Speech also has a range of frequencies, but it mostly limited to the range between a hundred (or so) hz and 8,000 (or so) hz. The frequencies that make up vowel sounds are typically lower frequencies, while the consonant sounds (at least the parts that help us hear which consonant sound was spoken) tend to be higher frequency sounds.
People with even moderately good hearing up to about 3,000 hz can understand speech fairly well. Wired telephones typically do not transmit sound above 3,500 hz.
When people have a hearing loss, they usually have less ability to hear some frequencies than to hear other frequencies. This creates a distortion that can make it difficult to understand speech. Usually, people with hearing loss have poorer hearing in high frequencies than in low frequencies, but some people may lose lower frequencies, or even middle frequencies, while having less loss in other frequencies.
Hearing aids now have the ability to be tuned to amplify different frequencies by differing amounts, so that it fills in where you need the amplification without giving you too much sound in frequencies where you hear better.
Thanks to Al Winney for suggesting this term.