5g (5th generation) is related to the frequency range used through cellular networks communication to and from your phone or to and from your Mi-Fi routet (the one with a sim card). Typically to bridge the gap from mobile device to your ISP.

Wi-Fi has 2.4G and 5G sellection bands on your home router for in-house wireless (internet) connection is actually describing the frequency range area (2.4G = 2.4GHz and 5G is 5GHz).

5g cellular uses way higher frequencies.

https://en.m.wikipedia.org/wiki/5G

And that frequency range can provide for faster data, but does not reach very far, hence the amount of repeater antennas that need to be installed in a city to reach everyone. As example on the wikipage you see those radio frequencies (RF) are way higher and the power required is also way higher than previous generation antennas (4g, 3g, etc.) Such that many people having such a repeater antenna close by start to worry about the effects on the human body.