In relation to 5G and 4G phones, this refers to how strong of a radio signal the phone is allowed to transmit when communicating with the cellular network (the uplink.)
The default is Power Class 3, which is 23 dBm. This is sufficient for most sub-6 GHz frequency bands. However higher frequencies benefit from higher power levels in some situations.
Therefore Power Class 2 allows higher power levels (up to 26 dBm) in certain bands, including bands 41 and 77. In relation to 4G LTE, this is commonly referred to as HPUE.
There is also an even stronger Power Class 1.5 for 5G, which allows up to 29 dBm for band 41.