antenna
An antenna is the bridge between a wire and the open air — a metal shape that turns a wiggling electric current into a radio wave that flies off at the speed of light, and vice versa. Picture a stone dropped in a pond: the splash makes ripples spread outward. An antenna is the splash, and the radio wave is the ripple. Your phone, Wi-Fi router, FM radio, and a satellite dish all hide one inside.
The trick is that a current oscillating back and forth in a conductor that is a sizeable fraction of a wavelength long stops storing its energy locally and instead launches it as a self-sustaining electromagnetic wave. Because of reciprocity, the very same piece of metal that transmits well also receives well — a single antenna does both jobs. Size scales with wavelength: a 2.4 GHz Wi-Fi antenna is a few centimetres, while an AM broadcast tower at 1 MHz must be hundreds of metres tall.
An antenna does not 'amplify' a signal in the way an amplifier does — it has no power source. Its 'gain' simply means it focuses the energy it already has into a preferred direction, like a flashlight reflector concentrating the same bulb's light.