One definition of impedance is the change in voltage divided by the change in current. If you know calculus, this can be written as Z=dv/di.
So, for instance, if you drive the base of a common emitter stage with 10uA dc, and you change tha current by 10nA, and this causes the base voltage to change by 25uV, then the input resistance (at this bias current) would be
Rin=25uV/10nA=2.5kohms.
This value changes as the base bias current changes.
If you do a similar measurement at high frequency where the input capacitance draws current, the input impedance would be a complex number, meaning it would have a magnitude and a phase angle.