Radio frequency bands below 6 GHz (6,000 MHz).
Prior to 2019, all radio frequency bands used for mobile phones were below 6 GHz.
5G introduced much higher-frequency mmWave bands, and thus the need for the term "sub-6 GHz" (sometimes just "sub 6") to distinguish existing bands from mmWave.
See: 5G NR
While mmWave bands can support much faster data, they can't be used for broad coverage over large areas. Sub-6 frequencies can provide broad coverage.
Within the sub-6 GHz range are both low-band and mid-band frequencies.
The higher-frequency "mid-band" ranges within sub-6 generally offer the faster data service that can be associated with 5G service.
In the US, the lower-frequency "low-band" sub-6 GHz bands are generally the same bands used for 4G. In many cases, a band will be sub-divided and/or use dynamic sharing technology so that both 4G and 5G technology can use in the same band. 5G in these bands is sometimes not much faster than 4G.