Due to legislation, the radio spectrum has been divided up to allocate specific bands to specific purposes. The common license for wireless audio equipment is Channel 38 (606.5–613.5 MHz).
Wireless audio operates on radio frequencies. Frequency denotes the channel (RF) where amplitude denotes the signal (AF).
There are available tools to find the best frequencies in the current area, including Shure's Frequency Finder which uses the records of other people to find frequencies, and Wireless Workbench.
The use of waves is always changing, meaning the wireless frequencies that work now may not in the future.