A radio frequency band near 28 GHz. It's a mmWave frequency band used for 5G in the US.
See: mmWave
See: 5G
Because mmWave bands are much higher-frequency than bands used for 4G and 3G, they do not reach as far, nor as well through solid objects. A typical mmWave base station ("tower") only covers one or two city blocks, at most.
However mmWave bands such as band 261 can offer much higher speeds than other, lower frequency bands.
Band 261 technically spans 27,500 – 28,350 MHz.