There are several reasons why your lights may dim when your AC comes on. There may be an overload in the circuits. There could be a circuit breaker malfunction. There could be a loose connection. There may be an issue with low voltage. The wiring in the AC may be undersized. There could be a resistance problem in the wiring. There may also be issues with the wiring in the home's electric system. Finally, there can be issues with the motor on the air conditioner. If this is something new that has just started occurring, you should have a licensed electrician come and check out the situation.