In the context of cellular phones and networks, mid-band radio frequencies are those that are between roughly 2 and 6 GHz.
Lower frequencies generally provide better coverage, while higher frequencies can accommodate more and/or faster data. Therefore mid-band provides a good balance between the two, ideal for 5G networks.
In the US, mid-band includes band 41 (around 2.5 GHz) and band 77 AKA C-Band (around 3.7 GHz).
See: Band 41
See: C Band