It means you can walk in, take whatever you want, and kill anyone who tries to stop.
Explanation: This joke satirizes the controversial legacy of Christopher Columbus. It humorously (though darkly) compares the “Columbus Day sale” to Columbus’s actions upon arriving in the Americas—claiming land and harming indigenous people. The punchline highlights this historical critique by suggesting that shoppers can take whatever they want and harm anyone in their way, paralleling the way Columbus’s arrival is viewed by some as an act of conquest and violence rather than discovery. The humor comes from the exaggerated and shocking nature of the comparison.