This is the impedance rating of the loudspeaker averaged across its frequency range in some manner. Sometimes it's just literally an average of the whole lot; sometimes it's an average of the values at specified points. Either way, it's what you think of when you hear talk of a "4 Ohm" or an "8 Ohm" loudspeaker.
Exactly what it says – the minimum value of the impedance For a conventional speaker this is usually just below the nominal impedance, so a nominal 4 Ohm speaker might have a minimum value of, say 3.2 Ohms. How low this figure is and where it happens in the frequency range are important for amplifier matching. Old Apogees used to go down to about 1 Ohm at the very low bass IIRC, which upset many amplifiers! Equally, electrostatics tend to hit their minimum impedance at high frequencies, which some amplifiers don’t like. In general, the lower the minimum impedance value, the more of a tricky load the speaker can be to drive.
Again, fairly self explanatory. If you have a conventional two way speaker with a crossover frequency of 500Hz, then the bass/mid driver is handling everything below 500Hz, and the tweeter is handling everything above. Obviously it's more complex than that, depending on the slope of the crossover, which is a measure of how quickly the circuit is stopping frequencies the 'wrong' side of the crossover point from reaching the driver.
It's something that sounds simple to work out. Trust me - it isn't!