reactance
A resistor opposes current the same no matter the frequency. A capacitor or inductor is different: how hard it pushes back depends on how fast the signal wiggles. Reactance is that frequency-dependent opposition, the resistance of a reactive part, measured in ohms but changing with frequency.
A capacitor's reactance is Xc = 1/(2 times pi times f times C). It falls as frequency rises, so a capacitor blocks DC (infinite reactance at 0 Hz) and passes high frequencies easily. An inductor is the opposite: Xl = 2 times pi times f times L rises with frequency, so a coil passes DC but chokes high frequencies. Example: a 1 uF capacitor at 1 kHz has Xc = 1/(2 times pi times 1000 times 0.000001), about 159 ohms; at 10 kHz it drops to about 16 ohms.
Reactance is the heart of every passive filter and tuned circuit: it lets a component favour some frequencies over others. Honest caveat: unlike a resistor, an ideal reactance dissipates no power (it stores energy then gives it back), and it always comes with a 90-degree phase shift, so reactance and resistance cannot simply be added as plain numbers. That is what impedance is for.
A coupling capacitor must be large enough that its reactance is small at the lowest audio frequency. At 20 Hz a 10 uF capacitor has Xc = 1/(2 times pi times 20 times 0.00001), about 796 ohms, fine driving a 47 kilohm input but too high for an 8 ohm speaker.
Reactance changes with frequency.
Reactance is measured in ohms but is not resistance. It stores and returns energy rather than dissipating it, and it is always paired with a 90-degree phase shift.