I am a student of physics. I do not have a deep foundation in electronics. Is there a simple way to understand what is meant by the *input resistance* and *output resistance* of a circuit (in particular, that of a transistor amplifier)?

I have read the two-port hybrid parameter analysis of the BJT amplifier in common emitter (CE) mode. I have derived the expression for the input resistance $$R\_i=h\_{ie}-\frac{h\_{re}h\_{fe}R\_L}{1+h\_{oe}R\_L}$$ and an even more complex expression for the output resistance $$R\_o=\frac{h\_{fe}R\_L}{h\_{ie}+(h\_{ie}h\_{oe}-h\_{fe}h\_{re}) R\_L}.$$

The derivation is fine! But I don't have a good intuition for these expressions. Physically, what do these expressions really mean? Do they represent some equivalent resistances? I so, the equivalent of what? Also with these expressions, I don't have any feel for why the input resistance must be very high and the output resistance must be very small, for a good amplifier.