The source impedance of a microphone is the equivalent total AC resistance to current flow that would be seen looking onto the microphone’s output. Ideally, a microphone should be connected with input impedance ten times greater than the microphone output impedance.
Microphones have two classes, high and low impedance. Most professional microphones are low impedance, meaning their source impedance is lower than 150 ohms. Piezoelectric contact pickups, guitar pickups and cheap microphones are usually high impedance, meaning their source impedance is 25K ohms or higher.
Low impedance microphones are preferred in live sound and recording because if they are connected properly, will not be as susceptible to signal noise. This is because the cable is passing more signal than voltage, so noise must have more energy to get into the circuit. Low impedance microphones must use a transformer when plugged into a higher impedance input to retain noise refection. The most important aspect of low impedance is that the signal can travel hundreds of feet, whereas a high impedance signal is limited to about 20 feet without amplification.
High impedance microphones and pickups require a transformer when used with low impedance inputs, or long cables. The transformer will drop the signal to low impedance to work with the connection. High impedance microphones can create a higher output signal voltage, which may be why they are often inexpensive. Another reason is that they to not have the cost of a transformer, or 3-pin XLR connection.
Impedance has nothing to do with price or quality however. It is just another factor to consider when choosing a microphone.