To really understand the difference of high/low impedance and appreciate its importance, you need to be familiar with some basic circuit configuration, such as diode-connected transistors, cascode transistors and some simple feedback configuration. It is not an easy problem, but it can not be too hard either.
Normally gate and drain are high impedance node, while source is typically low impedance. But it is not always true. If the transistor is diode connected, the impedance of drain and gate will be reduced.
For a two or three stage opamp, the highest impedance node is normally the output of gain stage. You could find that the compensation capacitor is located here in most of the cases.
Source is usually a low impedance node unless an ideal current source or a current mirror is used to bias the source. Gate is a high impedance node in common source configuration.
Drain is usually a high impedance node except if connected to diode-connected MOS load.