an antenna
An antenna is the doorway between a circuit and free space: it turns the back-and-forth current in a wire into an electromagnetic wave that flies off through the air, and on receive it does the reverse, catching a passing wave and turning it back into a tiny current. Think of it as a transducer for radio, the same role a loudspeaker plays for sound, converting between a guided electrical signal and an open-air wave. Without an antenna a transmitter just heats up; with a good one it reaches across the planet.
An antenna radiates best when its size matches the wavelength, which is why resonance is central. The classic example is the half-wave dipole: two straight conductors in a line, together about half a wavelength long, fed in the middle. At resonance the current and voltage stand in a natural pattern, the antenna's impedance becomes a convenient near-resistive value (about 73 ohms for an ideal dipole, close enough to feed with coax), and it radiates efficiently. A quarter-wave whip over a ground plane does the same job in half the height by letting the ground act as a mirror. Antennas also have gain and directivity: a plain dipole spreads energy fairly evenly, while a dish or an array of elements concentrates it into a beam, trading wide coverage for reach in one direction. Gain is not amplification, the antenna adds no power, it just focuses what it has, like a flashlight reflector aiming the same bulb's light into a beam.
Antennas are where every wireless link begins and ends, from the trace antenna in a key fob to a deep-space dish. Two honest points for a beginner. First, an antenna must be matched and resonant near the operating frequency to work well; the same metal is a fine antenna at one frequency and nearly useless at another, which is why antennas are cut to size. Second, antenna gain is a trade, not free power: more gain in one direction always means less in others, and a high-gain antenna must be aimed. Real antennas are also affected by everything nearby, ground, hands, metal, so the tidy textbook numbers shift in the real world.
For the 2.4 GHz WiFi band the wavelength is about 12.5 cm, so a half-wave dipole is about 6.25 cm and presents roughly 73 ohms, close to the 50 ohm system. Cut the same dipole for 900 MHz and it becomes about 16.5 cm, a different size for a different band.
A dipole is cut to about half a wavelength, so its size depends on the band.
Antenna gain is focusing, not amplification; the antenna adds no energy, so more gain in one direction means less in others. An antenna is sized for its band and works poorly at frequencies far from resonance.