means A large prize or cash payout, especially one that accumulates until won, or by extension any sudden stroke of great luck.

from Born at the poker table. In a 19th-century American game called 'jackpots,' the pot couldn't be opened until a player held a pair of jacks or better — so the stakes built and built, untouched, until someone finally had the cards to claim the whole swollen pile. From that growing, locked-up pot the word spilled out into slot machines and lotteries, and then into everyday speech as any windfall.