In jackpots, the seed is the amount a progressive jackpot resets to after it is won. The pot then grows again from that floor as contributions are added.
The seed is usually funded from earlier contributions or by the provider, so a jackpot never restarts at nothing. Some systems divide each contribution between the visible pot and a reserve that funds the next seed.
The word has a second meaning in software. A PRNG generates its sequence from a starting value called a seed. Gambling RNGs are seeded from unpredictable sources and checked by test houses so that the seed cannot be guessed and the results cannot be predicted.