Lee Drutman at Vox reports:

If Maine Question 5 passes, Mainers will get to select up to five candidates in order of preference. If there is no majority in the initial tally of voting, and their first choice finishes last in the initial tally, their vote will be transferred to their second choice in the next tally. (In each tally, the last-place finisher gets eliminated). And if there is still no majority, and their second choice ranks last in the second tally, their vote will be transferred to their third choice, and so on, until one candidate has a majority. Or, put another way: If one candidate wins a majority in the initial tally, there is no runoff. If no candidate wins a majority, candidates are eliminated from the bottom-up, with each eliminated candidate’s supporters going to their next-ranked choice for the following round, until one candidate has more than half of the votes. (For a video explanation of how this works, I recommend this short explainer.)

Versions of the Single Transferable Vote (STV) have been used in Ireland, Northern Ireland, Malta, Tasmania, and NYC City Council from 1937-1947

STV systems tend to make candidates more civil to each other, and less likely to attack each other’s character and ideology. More generally, it induces politicians to cater more frequently to constituency whims and preferences, at the expense of drawing sharp contrasts across ideologies. You don’t want the voters from the opposing ideology to rank you very low, so you’ll ease up on the insults and try to appear like a very useful centrist. The resulting emphasis on constituency service has historically been the case in Ireland (most but not all of the time). Similar tendencies have been observed in Tasmania and Malta, and the parties evolve to become less ideological.

Traditionally, I have not been a huge fan of STV systems, but this year they sound a bit better than usual.

You can start here on the literature on STV.