Tuned circuit
A coil and a capacitor joined together: electricity sloshes between them at one natural frequency, so the circuit picks out signals near that frequency.
A tuned circuit is a coil (an inductor) connected to a capacitor. Charge flows out of the capacitor, builds a magnetic field in the coil, and the collapsing field pushes the charge back again. The electricity sloshes back and forth at one natural frequency, f = 1 / (2π√(LC)), set by the size of the coil (L) and the capacitor (C).
Feed it radio waves of many frequencies and it responds strongly only to those near its own rhythm, just as a swing builds up when you push in time with it. Turning the dial on an old radio changes the capacitor and so moves that sweet spot from one station to the next.
Oliver Lodge patented the idea for wireless in 1897 and called it syntony. Early tuned receivers were not very sharp: an unwanted station a little off frequency was only turned down by a modest factor, perhaps tens of times.
A common misconception is that tuning blocks other signals completely. It only weakens them. A strong enough signal nearby will still get through, which is exactly how a magician spoiled Marconi's demonstration in 1903.
