When capacitors and inductors (coils) are connected to an AC voltage source, they oppose the flow of electrical current. The amount of opposition depends on the value of the component (capacitor or inductor) and the applied frequency.

Capacitors block direct current (DC) and tend to block lower frequencies. This is why they're useful in high-pass filters/crossovers.

Inductors pass low frequencies with little or no opposition and tend to block higher frequencies. This is why they are used to block mids and highs when connected in series with a low frequency speaker.

In theory a perfect inductor or capacitor has no resistance. The reactance of a capacitor or inductor is similar to the resistance of a resistor. The biggest difference is that a resistor essentially opposes AC equally at all frequencies. The reactance of a coil (inductor) or capacitor varies with frequency.