What is the impedance of a transistor?
Impedance is a theoretical concept - a pararameter in a mathematical model of amplifiers. A common theoretical model has total 4 impedances for an amplifier: Input impedance, output impedance, transfer impedance and feedback impedance. Those impedances are complex numbers in equations which describe how input signal current and voltage depend on each other and on signal current and voltage in output.

Transistor is a common part which can act as an amplifier. To design a proper circuit around a transistor to make a working amplifier one needs those theoretical impedances. The impedances depend on frequency and on the applied dc voltages and currents.The manufacturer publishes them in the datasheet of the transistor. 

This is especially important in radio circuits. Without a proper mathematical design  one cannot squeeze out the optimal amplification  from a transistor, probably he gets nothing if the frequency is 100 MHz or more.

If you want to learn that math, you should at first know thoroughly the general AC circuit theory and master the calculations with general complex impedances. Then you have a possiblity to understand the theoretical 2 port (or four pole) amplifier models. 

Then you must know and understand how amplifiers can be constructed using a transistor as the amplifying component. First using idealized transistors, then using practical transistors with practical radio frequency circuitry and apply those impedances (or equivalent other two port parameters) that the manufacturer has supplied. You apply the impedances to match the external circuitry with the transistor.

Word impedance is no shorcut to proper understanding of transistors. Impedance is far too general concept. As well you could go to a car shop and start "I want a car that has a good thickness, It must be good in and out. All I want is a car with a good thickness both in and out I'm not interested in anything else and I want to hear nothing else!"