Impedance is the resistance a speaker poses to the amplifier. The higher the impedance easier it is for the amp to drive the speakers. Speakers are generally 4, 6 or 8 ohms. So 8 ohms is easier to drive than a 4 ohm speaker.
Now the problem is the speakers even the ones rated at say 8 ohms would drop down to even 4 ohms in the real world. I believe some speakers even drop down to 1 ohm and then its a nightmare for the driving amp to remain stable.
When the amp power output is stated its generally stated at a 8 ohm speaker impedance and generally the power doubles as the impedance halves. So a 100 watts per channel amp @ 8 ohms should give 200 watts per channel @ 4 ohms.
Hope i am not confusing u. In short its better to buy speakers with a higher impedance rating.