How does an inductor block frequency?
Ideal inductor's impedance is given by:

$$Z_L = j\omega L = j2\pi f L$$

This means that its impedance (which can be interpreted as "current opposition") is proportional to signal frequency. Therefore it acts as short circuit to DC current (\$f = 0\$, and \$Z = 0\$) and its impedance increase linearly with frequency. The larger the inductance \$L\$, the "faster" this increase goes.

If you're interested in the physical background of this, such behavior model can be derived from Maxwell's equations, particularly from the third equation (Faraday's law of induction).

An example application of using inductors in filters is given in the following circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

This is a low-pass filter, with cut-off frequency given by

$$f_c = \frac{1}{2\pi\sqrt{LC}}$$

Other filter types can be obtained exploring the same properties (high-pass from switching L and C, bandpass with other topologies, etcetera).