Can you clarify reactance? What is it exactly?

Reactance is just the imaginary part of the impedance. A circuit's equivalent impedance $\hat{Z}$ can have contributions from resistive, inductive and capacitive components (which combine according to the resistor rules). Since in general $\hat{Z}$ has a real and imaginary part, it can be written $\hat{Z}=R+jX$. The real part $R$ is the ``resistance'', and $X$ is called the ``reactance'', since it results from having imaginary impedances from inductors and capacitors as part of the circuit. Inductors and capacitors are not like stodgy resistors in that they ``react'' to time-dependent sources.