A "paid out" slot machine refers to a slot machine that has paid out its jackpot or other winnings to a player. In other words, a paid out slot machine is one that has recently awarded a payout to a player and has a lower balance of coins or credits than it did before the payout.

phrasal verb\n\n : to find a place for (someone or something) in a schedule, plan, etc. I can slot you in at 2 p.m.