Frequency is important for a couple of reasons. Generally, lower frequency (12-kHz or less) metal detectors handle ground interference better, and hit hard on coin-sized or larger high conductor targets. Lower frequency machines do NOT detect deeper than higher frequency machines.
Metal detectors using frequencies in the mid-teens (14-kHz to16-kHz) are often thought of as more “All Around” machines. They can be used as coin and jewelry hunters, relics, and even gold nugget hunting in some cases.
Higher frequency metal detectors (17-kHz to 71-kHz) can detect much smaller targets that lower frequency machines will walk right over. A good high-frequency machine will hit on a BB-size target at 6” every time while a coin machine won’t even know it’s there. The added sensitivity to small targets comes at a price however, as higher frequencies experience depth loss on larger targets in highly mineralized soils.
Multi-frequency machines all respond like low frequency machines and make great coin and jewelry metal detectors, but they are not good with tiny targets or as gold nugget machines.