When statisticians want to predict a binary outcome (Hillary wins vs Hillary does not win), they imagine that the universe is tossing an imaginary coin - Heads, Hillary wins; tails, she loses. To some statisticians, the coin represents their degree of belief in the outcome; to others, the coin represents what might happen if we reran the election under the same circumstances over and over. Philosophically speaking, it's hard to know what we mean when we speak of uncertain future events, even before we drag numbers into it. But we can look at where the number comes from.
At this point in the election, we have a sequence of poll results. These are of the form: 1000 people were polled in, say, Ohio. 40% support Trump, 39% support Hillary, 21% are undecided. There would be similar polls from previous elections for the respective Democratic, Republican (and other trace party) candidates. For previous years, there are also outcomes. You might know that, say, candidates with 40% of the vote in a poll in July, won 8 out of the 10 previous elections. Or the results might say, in 7 out of 10 elections, Democrats took Ohio. You might know how Ohio compares to Texas (perhaps they never choose the same candidate) - you might have information on how the undecided vote breaks down - and you might have interesting models of what happens when a candidate begins to "surge". They might also look at who tends to actually get out and vote, and what happens if it's snowing on The Day.
So when you take previous elections into account, you can say that the election coin has already been tossed a number of times. The same election is not being rerun every 4 years, but we can pretend that it sort of is. From all this information, the pollsters build complex models to predict the outcome for this year.
Hillary's 75% chance of winning is relative to our state of knowledge "today". It's saying that a candidate with the kind of poll results she has "now", in the states that she has them, and given the trends in her polls throughout the campaign, wins the election in 3 election years out of 4. A month from now, her probability of winning will have changed, because the model will be based on the state of polls in August.
The US hasn't had a statistically large number of elections in its history, much less since polling began. Nor can we be sure that polling trends from, say, the 70's, still apply. So it's all a bit dodgy.
The bottom line is that Hillary should begin working on her inauguration speech.