From my understanding, a coherer was a radio-wave detector that used ferrite particles (in a small sealed glass tube, with wire at each end) that - when the radio signal was strong enough - would "stick together" and form a circuit; you can think of it as an RF-activated "reed switch" or relay. It was very crude, and not easily tunable.

Also - another interesting tidbit of info about early wireless: In order to generated the higher carrier-wave frequencies, they used special multi-pole alternators spun at high speed. Rather than only having a few poles, like a regular power generator, these RF alternators would use sometimes hundreds of poles. The RPM, multiplied by the number of poles, would determine the frequency of the output signal carrier wave. This was mainly used for telegraphy; I'm not sure if these machines had the bandwidth for voice. At the same time, they still produced a ton of power like a standard alternator - so it was essentially a way to generate "high power" (100s to 1000s of watts) RF (in the KHz ranges).

Another early technology of the time (used for a variety things - including motor control) was something called "magnetic amplifiers"; they are essentially weird variations on standard AC transformer technology. They can be made to act as audio amplifiers, as well as switches (like a vacuum tube or transistor).