Hertz is a measure of frequency. Specifically, it's one cycle per second.
Almost everything has a frequency. The earth has a frequency of spinning around its axis (once every 24 hours) and of travelling around the Sun (once per year). But the measure, Hertz, is more typically applied to things with frequencies of one or more Hertz.
Many things related to hearing loss are measured in Hertz (usually abbreviated, Hz). We typically use KiloHertz (KHz) to represent thousands of Hz, and MegaHertz (MHz) to represent millions of Hz. Now with very fast computers and even some phones, transmission is done in billions of Hz, or GigaHertz (GHz).
All the sounds we hear have a certain frequency. People with normal hearing can typically hear from about 50 Hz to about 20,000 hz (more typically 20 Khz). A 50 Hz sound would be like a low rumbling sound, and a 20 KHz sound would be very high frequency like the whooshing sounds of a cymbal. Most speech ranges from about a few hundred Hz to about 8 KHz, so you can lose a lot of high frequency hearing without serious impact to your ability to understand speech.
Many of the devices we use like hearing aids, and ALDs use radio to transmit a signal that represents sound. Radio receivers are designed to be able to hear certain frequency bands. FM ALDs typically use the 72-76 MHz band or the 216-217 MHz band.
Remote and some cell phones now use very high frequency to transmit their audio signals. You've probably heard of 900 MHz, 2.4 GHz and even higher frequencies when you go shopping for phones.
Many people using hearing aids also use radio (again each with their own frequency) to allow their hearing aid remote controls to issue commands to their hearing aids.
Some people with CROS or BiCROS hearing aids also use radio frequencies to transmit sound from one side of their head to the other side of their head.
It's important if you are hard of hearing to be aware of the term "Hertz" so you can understand how it affects you in all the ways it can touch your life. The main effect if things aren't choosen carefully is interference among various devices because they share frequencies ranges.