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" to distinguish existing bands from mmWave.
See: 5G NR
In the US, sub-6 GHz bands used for 5G are generally the same bands used for 4G and 3G. Some of these bands are split into 4G and 5G sub-divisions. More commonly, starting in late 2019, a technology called DSS (dynamic spectrum sharing) allows 4G and 5G technologies to share the same band, with allocations that can change rapidly to meet demand.