XL = reactance in ohms () f = frequency in hertz (Hz) L = inductance in henrys (H)
XL is small at low frequencies and large at high frequencies. For steady DC (frequency zero), XL is zero (no opposition), hence the rule that inductors pass DC but block high frequency AC.
For example: a 1mH inductor has a reactance of only 0.3 for a 50Hz signal, but when the frequency is higher at 10kHz its reactance is 63.