With IRV, each voter ranks the candidates in order of their choice: favorite candidate is number one, second-favorite is number two, and so on. 

In single-winner IRV, a candidate must have more than 50% of the votes in order to win. If a candidate gets more than half of the electorate’s first-choice votes then they win the election. If not, the candidate with the fewest votes is eliminated, and any voter who picked the eliminated candidate as their first choice will have their vote count for their second choice. The process continues until a candidate wins with more than half of the votes. 

For races with multiple winners, the threshold for winning a seat is lower but the process is the same. In a city council election with four open seats, for example, each candidate must earn more than 20% of the vote share to win.

You can learn more about How IRV works from FairVote.org.