The “invisible signals” are electromagnetic radiation, same as visible light. It’s just not a wavelength your eyes can see. That’s the same category as microwaves, X-rays, UV light, infrared, etc. – it’s all just colours of light your eyes can’t see.
For radio and wifi, the ELI5 is that the data being sent is broken up into 1’s and 0’s (for wifi) or a wave representing the sound (for radio) and emitted as a pulsed/ flashing signal that travels out from the source in all directions, just like any other “light” does. Think of it like using a lantern to send Morse code as dots and dashes using long and short flashes of light. Radio broadcast are doing the same thing, beaming out pulses of light (a colour of light we call “radio”) across the landscape from their transmitter. Anyone with the right set of “eyes” for this colour – an antenna, can pick up this flashing light and decode the message back into a sound wave or digital data or whatever.
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