Ohms are how impedance is measured. Speakers' resistance will change while they are playing music. The speaker's ohm rating is usually given when the speaker is at rest (the cone is not moving). The higher the ohm rating, the less resistance and efficiency you will find in a speaker. A 2-ohm speaker will play louder but not sound as good, whereas an 8-ohm speaker will play cleaner and sound better.
Syner, Jason. How to Install Automotive Mobile Electronic Systems.
Minneapolis: MBI Publishing Company, 2009. Print.