when voltage source is connected across an ideal inductor, the current through it is limited by self induced EMF( N.d(phi)/dt).. This EMF opposes its cause by Lenz's law.
When steady dc supply is connected, there is no change of flux and hence no induced EMF is developed and inductive reactance offered to steady DC supply is zero.
At the same time this induced EMF is proportional to rate of change of magnetic flux/voltage, hence the inductive reactance offered is also proportional to frequency of voltage source.
