Inductors and Capacitors
Each of the waveforms on the extreme left of this picture represents a range of many
frequencies. In the top picture, a capacitor connected in series will
block low frequencies but allow higher ones to pass.
In the second picture, the higher frequencies will be diverted to ground through the capacitor, leaving just the lower frequencies at the output.
With an inductor, the situation is reversed. The third picture shows that a coil in series will block the higher frequencies while allowing the lower frequencies to pass and the bottom picture shows the lower frequencies being diverted to ground through the coil, leaving the higher frequencies at the output.
An oscillator will, inevitably, produce a range of frequencies rather than just a single one as desired. By careful selection and adjustment of capacitors and coils, the unwanted frequencies can be removed.