PA systems take a lot of power. A guitar player can use 50 watts and sound very loud. A 50 watt PA is way under powered for most applications. A power amp of at least 300 to 400 watts or more is required in most cases. An ohm is a measurement of electrical resistance.
All speakers have an ohm rating.
Some power amp companies will rate their power amps at 2 ohms, meaning that the amp will perform to this level if you have speakers with a 2 ohm rating plugged into the amp. However, very few speakers are rated at 2 ohms. Most speakers are rated at 8 ohms. If this doesn't sound important, please note that a 600 watt power amp may only perform at 125 watts when two 8 ohm speakers are plugged into it. What bridged means is that you take the two stereo signals and join them together so that the power will only support one speaker.