I just set the bar, niggas fall under it like a limbo
Setting the bar is a commonly used idiom, meaning to set the standard. It’s easy for Drake to set the standard and his competition are under the bar he is setting like they are playing a game of limbo
Considerig the Momento reference just before, “limbo” might also have a double meaning: the dream space, or “limbo” in the movie Inception (both movies are directed by Christopher Nolan), is where you believe dreams are reality. These other rappers are dreaming if they think they can match Drizzy.
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