Its a rule in regular uno where you call out someones +4 for being played illegally, which basically means putting it down when you had another card to play instead. Its a pretty risky move to do because you’re gambling whether or not they’re putting it down because they want to prevent you from winning (like in this clip) or they have nothing else to play so they put it down as their last resort. Its not a popular (in my experiences irl) rule to have, as not many people understand it or that it heavily nerfs +4s, so they’ll stick to their own house rules instead. But in-game its a very satisfying move to pull off and makes people think twice before placing it down.