about the band's music...

[Blasterion]

well actually the guitars aren't wireless themselves ya know..

It's the same as a wireless microphone, except that the transmitter is built into the microphone.
With a bass/guitar, you plug the instrument into a transmitter which sends the signal to a receiver, that usually sits on top of the amplifier it's connected to ...
The transmitter is like a lil box usually worn on the waist... unlike what Ayu has on her waist, that lil decorated box, which is a receiver, where her in-ear monitor is plugged in to, so she can hear her voice and the music better while on stage.

and you can play electric guitars in the rain just fine... but it's just annoying getting wet while playing