Impedance is how much a device resists the flow of an AC signal, such as audio.
Impedance is similar to resistance which is how much a device resists the flow of a DC signal.
Both impedance and resistance are measured in ohms. When referring to microphones, low impedance is less than 600 ohms medium impedance is 600 ohms to 10,000 ohms, high impedance is greater than 10,000 ohms. Not any more. In the early part of the 20th century, it was important to match impedance. Bell Laboratories found that to achieve maximum power transfer in long distance telephone circuits, the impedances of different devices should be matched.