It's the output voltage for a given pressure at its diaphragm.
The reference is 1V per pascal and the mic sensitivity is -28db (dBV as the unit is voltage) relative to that.
1V scaled by -28dBV is 0.040V
So it's sensitivity is 40mV per pascal.
For reference, the common Shure SM58 has a sensitivity of -56dBV, 2mV per pascal.
(The softest sound detectable is limited by noise level of the mic or preamp).
Impedance wise, electret capsules like that could be considered current sinks, I suppose?
It's operating current is given as 0.5mA; the load/power feed resistor needs to be appropriate to give a voltage across the capsule somewhere within it's stated working range. That resistor is a major part of the overall mic output impedance.
It could be anything from 470 Ohms at the low end of the voltage range to 10K or more at 10V. The overall circuit impedance will change significantly with the range of resistance.
The input impedance of the next stage should be at a minimum ten times higher than the load resistor value & preferably more, to keep distortion to a reasonable level.