The impedance rating on your speakers (given in ohms) is almost always the nominal impedance - which is roughly just an average. The actual impedance at any given frequency can vary greatly. A speaker might have a 3 ohm impedance at 200 Hz, but a 32 ohm impedance at 8000Hz. So the nominal rating is just an average taken over the entire frequency range of the speaker.
A lower impedance means that the speaker will call for more amps from your amplifier. This is very rough, but it is generally easier to deliver more volts and tougher to deliver more amps.
You can think of it like water being pushed through a hose. The impedance is like the size of the hose. A big, fat hose does very little to prevent the flow of water - so that would be low impedance. A skinny hose will make it tougher for water to flow, so that would be high impedance.
Volts are like the water pressure. It doesn't take a lot of pressure to get a lot of water to flow through a big, fat hose. But it takes lots and lots of pressure to get a lot of water to flow through a skinny hose. For most amps, it's pretty easy to increase the "water pressure" (ie. volts), so a skinny hose (high impedance) isn't a problem for amplifiers in general.
Amps are like the actual amount of water. A big, fat hose lets lots of water through - even without a lot of pressure. A skinny hose doesn't let very much water through.
Think of the amount of water being like the number of electrons. Lots of amps means lots of electrons. So a big, fat pipe (low impedance) means lots of water (electrons/amps) even with low pressure (volts). It's tougher for amplifiers to deliver lots and lots of electrons - much tougher than simply increasing the water pressure (volts).
So, in a general sense, it is easier to power a speaker that has higher impedance.
The ohm rating is just basically an average, so there are several ways that your speaker could wind up with a 6ohm nominal impedance rating.
One way is that it has several drivers (tweeters, woofers) that are wired in both series and parallel. When things are wired in series, it's like making a longer hose without making the hose any wider. Thus, the impedance goes up - still the same width of hose, just longer. When things are wired in parallel, it's like adding a second hose right beside the first hose. So impedance goes down - you basically have a fatter pipe (two hoses instead of one), but they aren't any longer and they didn't get skinnier all of a sudden.
So when you have a combination of things wired in series AND parallel, you can get some different and a little bit more unusual impedance ratings by the end of it.
Another reason why your speakers might be rated at 6ohms instead of the more typical 4 or 8 ohms is because your speakers vary A LOT at different frequencies. Maybe your speaker dips WAY down low to just 1 or 2 ohms at certain frequencies, while at other frequencies, it stays around 8 or 16 ohms. Remember, the nominal rating is an average. So if your speaker has certain frequencies where it dips really low, that's going to lower the overall average and you'll wind up with a speaker that would normally get an 8ohm nominal rating, but due to those few really low dips, it winds up with an average impedance that is closer to 6 ohms or something.
In general, for the sake of your amplifier, it's best if the lowest dips in impedance at any given frequency stay above 3ohms or so. THX calls for no dips lower than 3.2 ohms at any point. That way, the amplifier never has to try and deliver a crazy number of electrons all of a sudden. Keep that hose fairly skinny and it's easier to control the flow of water
Hope that helps!