To give a simple example, consider flipping a coin. You observe HHH. Is this a fair coin? or a double-headed one? or a biased coin? Different theories describe these situations, and you could be asked to bet on them. Imagine you then further observe HHHH - making a total of HHHHHHH. This makes your estimate of the chances of the "double-headed coin" hypothesis go up. Other hypotheses may increase in probability too - but we are not troubled by there being an infinity of them, since we give extra weight to the simpler ones, using Occam's razor.