[M.B.E.]

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can someone explain what 500 behind is?

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It's money/chips that are not physically present at the table but nevertheless are "in play" for the player on subsequent hands.

For example, if you give $300 cash to a chip runner, proper procedure is for the chip runner to count it and confirm the amount to the dealer, who can announce "300 behind". That way, the other players will know that the player in question is going to receive chips that are in play on the current hand. If the player loses those chips before they arrive, the dealer keeps track of how much the player "owes", and the debt can be settled when the chip runner returns.