Have you ever wondered why a pendulum keeps the same oscillation rate or.... why, when you hit a cantilever, it oscillates at a constant frequency? Ruler on the edge of a desk.
What keeps the pendulum and cantilever wobbling at the same rate (or frequency) no matter how much the energy decays, is called resonance.
Electrical circuits use the same technique to amplify a small signal to a larger one - it happens to be called a tuned-circuit and is used extensively in radio receivers.
A capacitor "hates" change; if the voltage across it changes it tries to fight it by either taking current if the voltage rises or, by supplying current should the voltage start to fall. Basically you could regard it like a flywheel. A flywheel in an engine smoothes out the mechanical effects of the pistons and makes the engine run smoother.
On the other hand, there is a mechanical component that seems to do the opposite to a flywheel. In electronics, this is called an inductor. It is like an elastic band; if you pull it and let go there is a recoil and it stings your fingers.
Now, imagine a flywheel connected to a motor running at 2000rpm via an elastic band. The instant the motor starts nothing much happens to the flywheel but, gradually it starts to rotate and some time later it acquires a speed of 2000rpm. Ok so far?
What happens next? There is still energy stored in the elastic band (inductor) and this has to unravel (because of recoil) and it continues to accelerate the flywheel to 4000rpm at which point the motor starts to regress it back down to 2000rpm. By the time it drops to 2000rpm, the elastic band has tightened in the opposite direction and needs to keep forcing the flywheel (capacitor) to a lower speed and, after a while, the flywheel is momentarily at zero rpm. This keeps happening but of course friction has its way and eventually the flywheel settles at 2000rpm after several swings above and below 2000rpm.
It's a million miles away from the tuned circuit in a radio receiver it seems but, not really at all; if the motor ramped up and down in speed at the rate the elastic band and flywheel wanted to resonate at, profound speed changes would be seen on the flywheel. This is a tuned circuit in operation; small changes at the correct frequency result in large changes on the output.
Thus, a capacitor in conjunction with an inductor form a very effective signal amplifier for an antenna.
It's so much more than this weasley explanation too.