Jeff,
Without getting too technical, Ohms is the measure of resistance, a lower number is less resistance, a higher number is more. Typical music reproduction speaker cabs are generally rated at 8 ohms or 4 ohms, sometimes something else. If you are going to use one and oly one cab, you can specify a 4 ohm rating that will put out more sonic power. Of course, you must be sure your amp will work with 4 ohms. Some amps are designed only for 8 ohms, but most professional amps can handle 4 or 2 ohm loads too for more output. When you use 2x 8 ohm cabs, the total ohms will be 1/2, or equal to 4 ohms in parallel wiring (normal). Not only will the amp put out more sound, but also you get more speaker surface area for more wind. Choices, choices.
Kris