In straightforward terms, the lowest frequency of audio devices represents how strongly it can reproduce bass sounds – but it doesn't mean that everything will be audible. We know this may not seem very clear, so let's first understand what frequency range stands for.
Take, for instance, a pair of headphones with a frequency range of 20 to 20,000 Hz; expressed in hertz, these numbers indicate the spectrum of sound vibrations that the product can deliver. Frequencies up to roughly 256 Hz are considered bass vibrations (some experts consider different thresholds). What's crucial to grasp here is: the smaller the number, the deeper the bass. Midrange sound waves occupy the spectrum of 256 Hz to (approximately) 2,048 Hz, and from there onward are the treble sounds.
Keep in mind that a product's lowest frequency and overall frequency range aren't the only factors to take into account when examining audio quality. What's more, an audio device may have its lowest frequency way under 20 Hz, but bass rumbles below this level may not be audible for the listener (although you may be able to feel it). The reason is that the typical reach of human hearing is of 20 to 20,000 Hz, and our hearing abilities do, unfortunately, deteriorate as we age (although this mostly impacts treble sounds). To put it bluntly, the lowest frequency of your chosen audio product may not be audible to you, although these sounds will be reproduced.
Note that frequency range tests are not exactly standardized, and a manufacturer's results may not be the same if submitted to a different analysis. Still, if you're all about that bass, the lowest frequency is something to bear in mind when choosing a product.