As rocknhorse1 said, the horseshoe is the symbol for Ohms (it's an Omega from the ancient Greek alphabet).
So don't muck around inside the chassis unless you know what you're doing. There are components in there that act like huge high voltage batteries, and they're connected to all the interesting parts. There are voltages in there that can kill you even when the amp is switched off and un-plugged. Don't touch anything inside the amp if you're going to look at wire colours.
