Reactance is the opposition a capacitor or inductor presents to alternating current. It is measured in ohms like resistance, but it does not dissipate energy — it stores it and hands it back, a quarter cycle later.
The two behave in opposite directions with frequency. Capacitive reactance falls as frequency rises, so a capacitor blocks DC and passes high frequencies — which is the entire basis of coupling and decoupling. Inductive reactance rises with frequency, so an inductor passes DC and chokes off high frequencies.
Reactance is not resistance
Current through a resistor is in phase with the voltage across it, and the product is real power turned into heat. Current through a pure reactance is 90° out of phase, so the average power is zero. That is why a large filter capacitor across the mains draws current without consuming energy, and why power factor correction works.
Combining into impedance
Resistance and reactance do not add arithmetically. They combine as vectors:
A 100 Ω resistor in series with 100 Ω of reactance gives 141 Ω of impedance at 45°, not 200 Ω. Where both a capacitor and an inductor are present, their reactances subtract before this step, because they are 180° apart from each other.
Real parts have parasitics
Every capacitor has some series inductance from its leads, so above a self-resonant frequency it behaves as an inductor. Every inductor has winding capacitance and does the reverse. This is why a single large decoupling capacitor does not work at high frequency, and why boards use several values in parallel.