All slot machines are computers programmed with a game to give you a payout based on the outcome it has produced. Whether it is a row of three, five or seven cherries, the payout is based on the probability of that happening. All slots use a statistical algorithm that produces a random outcome based on the algorithm. It is not pre-programmed, that is why its called a game of chance. The higher the payout you can receive the lower the probability of getting that specific combination of symbols.
Progressive slots are slot machines connected together in a network. They will still payout on individual outcomes, but the jackpot prize is the highest payout of the machines combined. Statistically speaking your chances of winning are higher because of a larger number of people playing on this network of machines to contribute to the jackpot prize. The truth of statistical equations is that the probability of winning that jackpot prize is actually reduced because of the larger number of possible outcomes on the progressive slot network.
The progressive slot network does not need to be limited to a single casino. With computer networks they way they are, casinos can operate local networked slot machines as well as interlinked network of networks. Sounds complicated? It is not. It only means that a casino in Johannesburg, can offer a jackpot prize on a progressive slot, that is linked to another such network in another casino in Cape Town. The computers keep track of the games, and have regular updates on the network so that the jackpot prize in Johannesburg and Cape Town will be the same.
People can then see how the prize is increasing, typically by a small increment for every game played on the progressive slot machines. The prize can statistically pay out at any stage, but the higher the prize the more likely people are to keep playing in the hopes of winning that prize. The prize usually starts off at a substantial amount. The probability of winning is small, and yet the incentive is there to keep trying, and thus increasing the prize and your hopes of winning it.