Sensitivity is the sound pressure level produced at 1 meter from the speaker, for a 1Khz frequency, at 1W of power. This is normally found at the specifications page for your speaker. Example: the Kef R3 meta has a sound pressure level (SPL) of 87db for a voltage of 2.83V.
(be careful that if the impedance is not 8 Ohm, such as with the Kef R3, that sensitivity is still listed at 2.83V and not 2V)

The nominal impedance is normally found at the specifications page for your speaker.
Example: the Kef R3 meta has a standard impedance of 4 Ohm.
Note: the given impedance may be incorrect, due to manufacturers not obeying the standard IEC 60268-5 spec.

Minimum impedance is normally found at the specifications page for your speaker.
The lowest speaker impedance (rounded down) should be at worst 80% of the nominal impedance, per the standard IEC 60268-5.
If it is lower, your nominal impedance is probably rated too high.
Example: the Kef R3 meta has a minimum impedance of 3.2 Ohm, which is 80% of 4 Ohm.

The frequency range is normally found at the specifications page for your speaker.
Example: the Kef R3 meta has a frequency range at +/- 3dB of 58Hz - 28000Hz according to the vendor. However, in testing the lower range is more like 40 Hz. You should go with 40 Hz in that case.

The frequency range is normally found at the specifications page for your speaker.
Example: the Kef R3 meta has a frequency range at +/- 3dB of 58Hz - 28000Hz according to the vendor, so please enter 28000.

This is the distance from the speaker to the listening position in meters.
For a distance of 1 meter, the sound pressure will be equal to the sensitivity.
Every time the distance from the speaker doubles, the sound intensity drops by 6 dB and the sound pressure lowers by 3 dB.
This doubles the required Wattage to keep the same SPL.

The maximum output volume is the limit it will accept. Example: the Kef R3 meta has maximum output volume of 110 dB SPL.
Combined with the lowest impedance, this determines the maximum power it can draw from the amplifier without damage.
For the Kef R3, this is 110dB at 3.2 Ohm (25% worse than 4 Ohm). Given its sensitivity it has a powerdraw of 256W@111dB, which translates to 256W * (1/1.26) * 1,25 for 254W @ 110dB worst case.
However, this is at 1Khz. It can sustain this volume over the frequency range, so the worst case is at the bottom of the supported frequency range which in our case is 58Hz.