Reversed polarity is when the hot and neutral connections at a receptacle are wired backwards. The wiring in your home is color coded. The black wire is "hot" or electrically charged. The white wire is the "neutral" wire and is not electrically charged. The black or hot wire should be connected to the brass colored screw and the white neutral to the silver screw.
If they are wired backwards, the appliance that you plug in will still work, but you have created a shock hazard. The reason that the receptacle slots on your outlets are different sizes is to prevent the neutral from going into the "hot" slot and vice versa. It must be important not to reverse the polarity.
If you plugged a lamp into an outlet with reversed polarity and decided to touch the metal light bulb socket, you would be shocked. It would be just fine in an outlet wired correctly.
At GEN3, we see this fairly often, especially on home inspection reports and when we do electrical inspections.
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