yea bcae1.com is what you need; but in short, an Ohm is the unit of measure of resistance...or the resistivity to the flow of electrons in a circuit. Ohm's law, which you will also learn about, says that voltage equals the amount of current times the amount of resistance. Lastly, power (watts) is defined as voltage times current. So you can see that the more resistance there is, the less amount of current there is (if voltage was steady). And less current means less power.
Speakers & subs present a "load" to an amp...which is measured in Ohms. The more Ohms there are, the less amount power the amp can produce. A lower amount of Ohms means more power. You will see the word impedance. For now, just think that impedance = resistance, and when you've felt you've learned enough about the whole ordeal, allow yourself to understand the difference between impedance & resistance.