Simply put, impedance is the resisting property of a coil. The lower the impedance, the easier it is for an amp to supply it with power. If the impedance is too low, however, it can lead to an amplifier to overheat. In the best case, the amp will shut down and cool off. Worst case, the amp burns out and is ruined.
What this means for you is that you need to match your sub’s impedance with your amp’s. Most amps can work with a 4 Ohm load. They can also work with 2 Ohm loads on each channel. So check the specs on your amp and subs before you make a purchase.