"Hard to drive" means one or both of the following:
1. At some audio frequencies but not all, the speaker requires a lot of power from the amplifier. If your amplifier can't deliver enough current at those frequencies, then sonically your system will "run out of gas" at louder listening levels. People usually refer to this as a "bumpy impendance curve." Your amplifier is working harder and running hotter when powering a speaker like this, which can shorten the amplifier's lifespan. There are a few infamous speakers that have very low impedance at all frequencies, such as the infamous Apogee Scintilla (nominal impedance of 1 ohm).
2. Some speakers have low sensitivity, requiring a LOT of amplifier power at all
frequencies to play at reasonable volume levels. The hardest-to-drive speakers are typically the ones with a very bumpy impedance curve and
for a really clear explanation that non-engineers like me can follow.
I would add that an amplifier has lots more power available for fronts/center/surrounds when the low frequencies are routed to the subwoofer's amplifier, as they are in your home theater system.
website, which I hadn't seen before, has some clearly written articles on the basics of system setup in it's "How To" section, under "Resources." You won't agree with everything there, e.g., whether cables benefit from break-in (the writer says YES).