When you push on the gas pedal in a semi your battery light come on just wonder if anybody know what this means?

That light indicates a fault in your charging system, and it works by monitoring the voltage in the system. Any semi made today has a 12-volt electrical system. They used to have 24-volt systems (the extra 12 volts made it easier to crank a big engine) but now that powerful 12-volt starters have been invented, trucks are 12v. Which means the system thinks the voltage in it should be between 11 and 13 volts. If it sees a voltage outside that range in either direction, the light comes on.

Since the light comes on when you step on the fuel pedal, the voltage is too high. Too high voltage means your voltage regulator is shot...since it's in the alternator, you need to pull it, take it to an auto electric shop and get it rebuilt. More importantly, you need to get it done NOW - pumping 18v into a 12v electrical system fries things, and the cheapest part in the system is the alternator. You don't want to know what an engine computer on one of these rigs costs.