I'm guessing effective output level is referring to a figure in mV/Pa. If so, this is often called sensitivity and is a measure of the voltage generated by a given amount of sound pressure. The higher this number, the more sensitive the mic is, and the less gain needed on a preamp to generate the same signal level compared to a less sensitive mic.
I'd need someone else to explain output impedance, though I have a a general idea.
Frequency response is a measure of how strong the signal is from a microphone at a given frequency. Mics that "roll off" low frequencies, for example, might need a bass boost applied to the signal.
Maximum SPL input is the level at which the microphone diaphragm will physically clip. This means that the mic stops increasing its signal output even though the SPL continues to increase. The result is a waveform that I'd suspect resembles a square wave.