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When the seller of an option has to carry out the contract. A call seller must sell 100 shares per contract at the strike price. A put seller must buy 100 shares per contract at the strike price. It usually happens when the stock ends past the strike on the last day: above it for a call, below it for a put. In real markets it can also happen early, but here paper options you sold are only assigned when they expire.
Example:
You sold 1 put with a $40 strike and the stock ends at $35. You must buy 100 shares for $4,000, though they are worth $3,500.