In this use, the term "to key" means to turn on and off a carrier, typically, e.g. it is said that one "keys the transmitter" by interrupting some stage of amplification with, for example, a telegraph key.
In this use, the term "to key" means to turn on and off a carrier wave, typically. For example, it is said that one "keys the transmitter" by interrupting some stage of amplification with a telegraph key.