Frequency

Frequency is just how fast a wave wiggles — cycles per second, measured in hertz. In radio the numbers get large quickly, so you live in kHz, MHz and GHz. A "3.5 GHz" carrier is oscillating three and a half billion times a second.

The practical consequence engineers care about is the trade-off it sets up. Higher frequency means shorter wavelength, which lets you pack more antennas into a small array (great for massive MIMO and beamforming) and opens up wider channels for more data. The cost is range: higher frequencies attenuate faster and are blocked more easily by walls, rain and even foliage. That single relationship — more frequency, more capacity, less reach — explains most of why network layers are arranged the way they are.